drm/amdgpu/gfx: add amdgpu_gfx_off_ctrl_immediate()

Same as amdgpu_gfx_off_ctrl(), but without the delay
for gfxoff disallow.

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Suggested-by: Błażej Szczygieł <mumei6102@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>