drm/amd/amdgpu : Fix NULL pointer comparison

Replace direct comparisons to NULL i.e.
'x == NULL' with '!x'. As per coding standard.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Ravikant B Sharma <ravikant.s2@samsung.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>