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>