drm/ttm: remove TTM_MEMTYPE_FLAG_MAPPABLE

Not used any more. And it is bad design to use a TTM flag
to do a check inside a driver.

Signed-off-by: Christian König <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/378245/

include/drm/ttm/ttm_bo_driver.h

···
 #define TTM_MAX_BO_PRIORITY	4U
 
 #define TTM_MEMTYPE_FLAG_FIXED         (1 << 0)	/* Fixed (on-card) PCI memory */
-#define TTM_MEMTYPE_FLAG_MAPPABLE      (1 << 1)	/* Memory mappable */
 
 struct ttm_mem_type_manager;