drm/etnaviv: Fix page property being used for non writecombine buffers

In the etnaviv_gem_vmap_impl() function, the driver vmap whatever buffers
with write combine(WC) page property, this is incorrect. Cached buffers
should be mapped with the cached page property and uncached buffers should
be mapped with the uncached page property.

Fixes: a0a5ab3e ("drm/etnaviv: call correct function when trying to vmap a DMABUF")
Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>