drm/ttm: Include <linux/vmalloc.h> to fix MIPS build

drm/ttm: Include <linux/vmalloc.h> to fix MIPS build

On MIPS, vmap() and vunmap() are undeclared in ttm_bo_util.c. An
error message is shown below.

  CC      drivers/gpu/drm/ttm/ttm_bo_util.o
  ../drivers/gpu/drm/ttm/ttm_bo_util.c: In function 'ttm_bo_kmap_ttm':
  ../drivers/gpu/drm/ttm/ttm_bo_util.c:364:32: error: implicit declaration of function 'vmap'; did you mean 'kmap'? [-Werror=implicit-function-declaration]
    364 |                 map->virtual = vmap(ttm->pages + start_page, num_pages,
	|                                ^~~~
	|                                kmap
  ../drivers/gpu/drm/ttm/ttm_bo_util.c:364:30: warning: assignment to 'void *' from 'int' makes pointer from integer without a cast [-Wint-conversion]
    364 |                 map->virtual = vmap(ttm->pages + start_page, num_pages,
	|                              ^
  ../drivers/gpu/drm/ttm/ttm_bo_util.c: In function 'ttm_bo_kunmap':
  ../drivers/gpu/drm/ttm/ttm_bo_util.c:429:17: error: implicit declaration of function 'vunmap'; did you mean 'kunmap'? [-Werror=implicit-function-declaration]
    429 |                 vunmap(map->virtual);
	|                 ^~~~~~
	|                 kunmap
  ../drivers/gpu/drm/ttm/ttm_bo_util.c: In function 'ttm_bo_vmap':
  ../drivers/gpu/drm/ttm/ttm_bo_util.c:509:23: warning: assignment to 'void *' from 'int' makes pointer from integer without a cast [-Wint-conversion]
    509 |                 vaddr = vmap(ttm->pages, ttm->num_pages, 0, prot);
	|                       ^

Fix this by including <linux/vmalloc.h>.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230116121023.14245-1-tzimmermann@suse.de