backend-drm/state-propose: Amend an older comment

Since b38b735e, 'backend-drm: Remove Pixman conditional
for keep_buffer' the Pixman renderer keeps its own reference to buffers
when attached to surfaces, rather than flipping keep_buffer variable for
the surface. Problem is that when switching from the Pixman render to
the GL would not work and could result in a crash upon first repaint.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>