drm/amd/display: remove unused variable res_pool

With gcc and W=1, there is this error
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_mst_types.c:1214:31:
  error: variable ‘res_pool’ set but not used [-Werror=unused-but-set-variable]
 1214 |         struct resource_pool *res_pool;
      |                               ^~~~~~~~

Since res_pool is unused, remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>