minigbm: virtio_gpu: set vma->length when mmap.

minigbm: virtio_gpu: set vma->length when mmap.

Othewise bo_unmap won't work. Also add a ifdef guard for PAGE_SIZE
to make it also compile under ARC++.

BUG=b:70179880
TEST=Run /usr/local/autotest/bin/screenshot.py with qemu+virgl
TEST=Run Android App under ARC++ with qemu+virgl

Change-Id: I586446c56da091dd82d9bc9c1010d9b4ee4d92ee
Reviewed-on: https://chromium-review.googlesource.com/959527
Commit-Ready: Lepton Wu <lepton@chromium.org>
Tested-by: Lepton Wu <lepton@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>