drm/amdgpu: Block MMR_READ IOCTL in reset

Register access from userspace should be blocked until
reset is complete.

Signed-off-by: Victor Skvortsov <victor.skvortsov@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>