drm/amd/display/dc/dce/dce_opp: Make local functions and ones invoked by reference...

drm/amd/display/dc/dce/dce_opp: Make local functions and ones invoked by reference static

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

 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:427:6: warning: no previous prototype for ‘dce60_opp_set_clamping’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:548:6: warning: no previous prototype for ‘dce60_opp_program_bit_depth_reduction’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:571:6: warning: no previous prototype for ‘dce60_opp_program_clamping_and_pixel_encoding’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:681:6: warning: no previous prototype for ‘dce60_opp_program_fmt’ [-Wmissing-prototypes]

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: Mauro Rossi <issor.oruam@gmail.com>
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>