drm/amd/display: Use NULL instead of 0

Silence the following sparse warnings:

../drivers/gpu/drm/amd/amdgpu/../display/dc/dce112/dce112_resource.c:865:16:
sparse: warning: Using plain integer as NULL pointer

../drivers/gpu/drm/amd/amdgpu/../display/dc/dce110/dce110_hw_sequencer.c:1588:84:
sparse: warning: Using plain integer as NULL pointer

../drivers/gpu/drm/amd/amdgpu/../display/dc/dce110/dce110_hw_sequencer.c:2725:84:
sparse: warning: Using plain integer as NULL pointer

../drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_resource.c:1919:16:
sparse: warning: Using plain integer as NULL pointer

v2: drop removal of default case to avoid adding warnings (Alex)

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Magali Lemes <magalilemes00@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>