drm/amd/display: Simplify bool comparison

Fix the following coccicheck warning:
./drivers/gpu/drm/amd/display/dc/dml/dcn21/display_mode_vba_21.c:5121:14-38:
WARNING: Comparison to bool

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <abaci-bugfix@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>