drm/etnaviv: etnaviv_cmdbuf.c: Drop the unneeded include of drm_mm.h

The etnaviv_cmdbuf.c doesn't reference any functions or data members
defined in drm_mm.h, remove unneeded headers may reduce kernel compile
times.

Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

drivers/gpu/drm/etnaviv/etnaviv_cmdbuf.c

@@ -5,8 +5,6 @@
 
 #include <linux/dma-mapping.h>
 
-#include <drm/drm_mm.h>
-
 #include "etnaviv_cmdbuf.h"
 #include "etnaviv_gem.h"
 #include "etnaviv_gpu.h"