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>