drm/doc: Remove solved "VBlank issues"

Remove the list of broken tests on VKMS solved by patchset
https://patchwork.freedesktop.org/series/55994/



Signed-off-by: Shayenne Moura <shayenneluzmoura@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190206193157.3b53ipdxtcqc4hv4@smtp.gmail.com