drm/amdgpu/uapi: Introduce AMDGPU_GEM_DOMAIN_MMIO_REMAP

Add a new GEM domain bit AMDGPU_GEM_DOMAIN_MMIO_REMAP to allow
userspace to request the MMIO remap (HDP flush) page via GEM_CREATE.

- include/uapi/drm/amdgpu_drm.h:
  * define AMDGPU_GEM_DOMAIN_MMIO_REMAP
  * include the bit in AMDGPU_GEM_DOMAIN_MASK

v2: Add early reject in amdgpu_gem_create_ioctl() (Alex).

Cc: Christian König <christian.koenig@amd.com>
Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>