drm/ttm: make TTM_MAX_BO_PRIORITY unsigned

Fix a warning about different types in min() macro in amdgpu:

In file included from ./include/linux/list.h:8:0,
                 from drivers/gpu/drm/amd/amdgpu/amdgpu_object.c:32:
drivers/gpu/drm/amd/amdgpu/amdgpu_object.c: In function ‘amdgpu_bo_create_restricted’:
./include/linux/kernel.h:739:16: warning: comparison of distinct pointer types lacks a cast
  (void) (&min1 == &min2);   \
                ^
./include/linux/kernel.h:742:2: note: in expansion of macro ‘__min’
  __min(typeof(x), typeof(y),   \
  ^~~~~
drivers/gpu/drm/amd/amdgpu/amdgpu_object.c:434:21: note: in expansion of macro ‘min’
  bo->tbo.priority = min(bo->tbo.priority, TTM_MAX_BO_PRIORITY - 1);
                     ^~~

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