modesetting: workaround for stride/pitch mismatch

This helps for the cases where buffer stride is miscalculated, the most
notable example being 1366x768 resolution. From what I understand the code,
the stride calculation should distinguish between any pixmap and the one
used for FB, which is not the case and everything is calculated in
glamor/glamor.c:227.

Fixes: xorg/xserver#889
Signed-off-by: Aleksander Miera <ammiera@hotmail.com>
3 jobs for stride-mismatch-workaround in 5 minutes and 40 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Docker Image
passed #4154945
debian-testing

00:00:22

 
  Build And Test
passed #4154946
autotools-build-and-test

00:05:18

passed #4154947
meson-build-and-test

00:00:50