drm/amd/display: fix comparison pointer to bool warning in dc.c

drm/amd/display: fix comparison pointer to bool warning in dc.c

Fixes coccicheck warning:
drivers/gpu/drm/amd/display/dc/core/dc.c:1543:12-19: WARNING: Comparison
to bool
drivers/gpu/drm/amd/display/dc/core/dc.c:1496:14-42: WARNING: Comparison
to bool
drivers/gpu/drm/amd/display/dc/core/dc.c:971:15-48: WARNING: Comparison
to bool
drivers/gpu/drm/amd/display/dc/core/dc.c:976:15-44: WARNING: Comparison
to bool

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>