drm/ttm: fix missed conversion of set_pages_array_uc

drm/ttm: fix missed conversion of set_pages_array_uc

This patch fixed the error when do not configure CONFIG_X86, otherwise, below
error will be encountered.

All errors (new ones prefixed by >>):

   drivers/gpu/drm/ttm/ttm_page_alloc_dma.c: In function 'ttm_set_pages_caching':
>> drivers/gpu/drm/ttm/ttm_page_alloc_dma.c:272:7: error: implicit declaration of function 'set_pages_array_uc'; did you mean
+'ttm_set_pages_array_uc'? [-Werror=implicit-function-declaration]
      r = set_pages_array_uc(pages, cpages);
          ^~~~~~~~~~~~~~~~~~
          ttm_set_pages_array_uc
   cc1: some warnings being treated as errors

Reported-by: kbuild test robot <lkp@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>