savage_drm.h: include <drm/drm.h>

Fixes compiler error:

drm/savage_drm.h:50:24: error: array type has incomplete element type
  struct drm_tex_region texList[SAVAGE_NR_TEX_HEAPS][SAVAGE_NR_TEX_REGIONS +

Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>