etnaviv: remove number of pixel pipes validation

etnaviv: remove number of pixel pipes validation

This validation was added before the etnaviv drm driver landed in
the linux kernel. Due some pre-merge API changes we had to fix-up
this value but with a mainline kernel this is not a problem anymore.

Lets remove that validation which also gets rid of problem caught
by Coverity, reported to me by imirkin.

Cc: "17.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>