drm/amdgpu:fix firmware memoryleak(v2)

this fix memory leak due to request_firmware after driver
unloaded

v2:
release gmc firmware for gmc6/7/8 as well

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>