drm: panel: fix excessive stack usage in td028ttec1_prepare

With gcc -O3 in combination with the structleak plug, the compiler can
inline very aggressively, leading to rather large stack usage:

drivers/gpu/drm/panel/panel-tpo-td028ttec1.c: In function 'td028ttec1_prepare':
drivers/gpu/drm/panel/panel-tpo-td028ttec1.c:233:1: error: the frame size of 2768 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]
 }

Marking jbt_reg_write_*() as noinline avoids the case where
multiple instances of this function get inlined into the same
stack frame and each one adds a copy of 'tx_buf'.

The compiler is clearly making some bad decisions here, but I
did not open a new bug report as this only happens in combination
with the structleak plugin.

This fixes mmtom ("init/Kconfig: enable -O3 for all arches")

Link: https://lore.kernel.org/lkml/CAK8P3a3jAnFZA3GFRtdYdg1-i-oih3pOQzkkrK-X3BGsFrMiZQ@mail.gmail.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org> [fix indent]
Link: https://patchwork.freedesktop.org/patch/msgid/20200108135116.3687988-1-arnd@arndb.de

drivers/gpu/drm/panel/panel-tpo-td028ttec1.c

@@ -86,7 +86,12 @@ struct td028ttec1_panel {
 
 #define to_td028ttec1_device(p) container_of(p, struct td028ttec1_panel, panel)
 
-static int jbt_ret_write_0(struct td028ttec1_panel *lcd, u8 reg, int *err)
+/*
+ * noinline_for_stack so we don't get multiple copies of tx_buf
+ * on the stack in case of gcc-plugin-structleak
+ */
+static int noinline_for_stack
+jbt_ret_write_0(struct td028ttec1_panel *lcd, u8 reg, int *err)
 {
 	struct spi_device *spi = lcd->spi;
 	u16 tx_buf = JBT_COMMAND | reg;
@@ -105,8 +110,9 @@ static int jbt_ret_write_0(struct td028ttec1_panel *lcd, u8 reg, int *err)
 	return ret;
 }
 
-static int jbt_reg_write_1(struct td028ttec1_panel *lcd,
-			   u8 reg, u8 data, int *err)
+static int noinline_for_stack
+jbt_reg_write_1(struct td028ttec1_panel *lcd,
+		u8 reg, u8 data, int *err)
 {
 	struct spi_device *spi = lcd->spi;
 	u16 tx_buf[2];
@@ -128,8 +134,9 @@ static int jbt_reg_write_1(struct td028ttec1_panel *lcd,
 	return ret;
 }
 
-static int jbt_reg_write_2(struct td028ttec1_panel *lcd,
-			   u8 reg, u16 data, int *err)
+static int noinline_for_stack
+jbt_reg_write_2(struct td028ttec1_panel *lcd,
+		u8 reg, u16 data, int *err)
 {
 	struct spi_device *spi = lcd->spi;
 	u16 tx_buf[3];