drm/amdgpu: remove unneeded conversions to bool

Found with scripts/coccinelle/misc/boolconv.cocci.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>