drm/amdgpu: call release_firmware() without a NULL check

The release_firmware() function is NULL tolerant so we do not need
to check for NULL param before calling it.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>