drm/amdgpu: Drop MMIO_REMAP domain bit and keep it Internal

"AMDGPU_GEM_DOMAIN_MMIO_REMAP" - Never activated as UAPI and it turned
out that this was to inflexible.

Allocate the MMIO_REMAP buffer object as a regular GEM BO and explicitly
move it into the fixed AMDGPU_PL_MMIO_REMAP placement at the TTM level.

This avoids relying on GEM domain bits for MMIO_REMAP, keeps the
placement purely internal, and makes the lifetime and pinning of the
global MMIO_REMAP BO explicit. The BO is pinned in TTM so it cannot be
migrated or evicted.

The corresponding free path relies on normal DRM teardown ordering,
where no further user ioctls can access the global BO once TTM teardown
begins.

v2 (Srini):
- Updated patch title.
- Drop use of AMDGPU_GEM_DOMAIN_MMIO_REMAP in amdgpu_ttm.c. The
  MMIO_REMAP domain bit is removed from UAPI, so keep the MMIO_REMAP BO
  allocation domain-less (bp.domain = 0) and rely on the TTM placement
  (AMDGPU_PL_MMIO_REMAP) for backing/pinning.
- Keep fdinfo/mem-stats visibility for MMIO_REMAP by classifying BOs
  based on bo->tbo.resource->mem_type == AMDGPU_PL_MMIO_REMAP, since the
  domain bit is removed.

v3: Squash patches #1 & #3

Fixes: 0561324837 ("drm/amdgpu/uapi: Introduce AMDGPU_GEM_DOMAIN_MMIO_REMAP")
Fixes: 2a7a794eb8 ("drm/amdgpu/ttm: Allocate/Free 4K MMIO_REMAP Singleton")
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Leo Liu <leo.liu@amd.com>
Cc: Ruijing Dong <ruijing.dong@amd.com>
Cc: David (Ming Qiang) Wu <David.Wu3@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
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>

drivers/gpu/drm/amd/amdgpu/amdgpu_object.h

@@ -168,8 +168,6 @@ static inline unsigned amdgpu_mem_type_to_domain(u32 mem_type)
 		return AMDGPU_GEM_DOMAIN_OA;
 	case AMDGPU_PL_DOORBELL:
 		return AMDGPU_GEM_DOMAIN_DOORBELL;
-	case AMDGPU_PL_MMIO_REMAP:
-		return AMDGPU_GEM_DOMAIN_MMIO_REMAP;
 	default:
 		break;
 	}