drm/etnaviv: etnaviv_cmdbuf.c: Drop the unneeded 'etnaviv_perfmon.h' header

The etnaviv_perfmon.h header is about performance monitor, it can hardly
has something to do with command buffer suballocator. Drop it.

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