drm/i915/ttm: tweak priority hint selection

For some reason we are selecting PRIO_HAS_PAGES when we don't have
mm.pages, and vice versa.

v2(Thomas):
  - Add missing fixes tag

Fixes: 213d5092 ("drm/i915/ttm: Introduce a TTM i915 gem object backend")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220209111652.468762-1-matthew.auld@intel.com