nv50,nvc0: fix buffer clearing to respect engine alignment requirements

nv50,nvc0: fix buffer clearing to respect engine alignment requirements

It appears that the nvidia render engine is quite picky when it comes to
linear surfaces. It doesn't like non-256-byte aligned offsets, and
apparently doesn't even do non-256-byte strides.

This makes arb_clear_buffer_object-unaligned pass on both nv50 and nvc0.

As a side-effect this also allows RGB32 clears to work via GPU data
upload instead of synchronizing the buffer to the CPU (nvc0 only).

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu> # tested on GF108, GT215
Tested-by: Nick Sarnie <commendsarnex@gmail.com> # GK208
Cc: mesa-stable@lists.freedesktop.org