drm: Include prototype for drm_need_swiotlb()

Sparse is not happy:
../drivers/gpu/drm/drm_memory.c:159:6: warning: symbol 'drm_need_swiotlb' was not declared. Should it be static?

Include the correct header for drm_need_swiotlb() prototype.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190710125143.9965-3-ville.syrjala@linux.intel.com
Reviewed-by: Sean Paul <sean@poorly.run>