glamor: an FBO is not needed for Xv pixmaps

It appears that on some hardware/diver combo such as nv30/nouveau, using
GL_ALPHA as format for 8-bit depth will cause an incomplete attachment
error (GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT) when trying to bind the
texture.

As a result, the FBO is NULL and glamor segfaults when trying to access
the FBO width/height in pixmap_priv_get_scale() in glamor_xv_render().

This happens with glamor-xv which uses 8-bit pixmaps, meaning that on
such hardware/driver, trying to play a video using Xv will lead to a
crash of the Xserver. This affects Xwayland, Xephyr, modesetting driver
with glamor accel.

But the use of an FBO is not actually needed for glamox-xv, so by
disabling FBO at pixmap creation, we can avoid the issue entirely.

Fix suggested by Eric Anholt <eric@anholt.net>

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=100710
Fixes: https://bugzilla.redhat.com/1412814


Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 7bfb87a2)