drm/amdgpu/ttm: Initialize AMDGPU_PL_MMIO_REMAP Heap

drm/amdgpu/ttm: Initialize AMDGPU_PL_MMIO_REMAP Heap

Add a one-page TTM range manager for AMDGPU_PL_MMIO_REMAP via
amdgpu_ttm_init_on_chip(). This only registers the placement with TTM;
no BO is allocated in this patch.

The singleton 4K remap BO is created and freed in the following patch.

This split follows to separate heap bring-up from BO allocation.

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