drm/amdgpu: implement AMDGPU_VA_OP_CLEAR v2

A new VM operation to remove all mappings in a range.

v2: limit unmapped area as noted by Jerry

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