drm/amdgpu: use memcpy_to/fromio for UVD fw upload

Also use the firmware size from the header instead of calculating it.

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