drm/amd/display/dc/inc/hw/dpp: Mark 'dpp_input_csc_matrix' as __maybe_unused

'dpp_input_csc_matrix' is used by some, but not all source files which
include dpp.h.

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/../display/dc/inc/hw/dpp.h:50:42: warning: ‘dpp_input_csc_matrix’ defined but not used [-Wunused-const-variable=]

NB: Snipped lots of these for brevity

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/inc/hw/dpp.h

@@ -47,7 +47,7 @@ struct dpp_input_csc_matrix {
 	uint16_t regval[12];
 };
 
-static const struct dpp_input_csc_matrix dpp_input_csc_matrix[] = {
+static const struct dpp_input_csc_matrix __maybe_unused dpp_input_csc_matrix[] = {
 	{COLOR_SPACE_SRGB,
 		{0x2000, 0, 0, 0, 0, 0x2000, 0, 0, 0, 0, 0x2000, 0} },
 	{COLOR_SPACE_SRGB_LIMITED,