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>
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