glamor: make sure the correct FBO is cleared

This also removes an unnecesary call to glDrawBuffer.

Signed-off-by: Dor Askayo <dor.askayo@gmail.com>
Fixes: 0e9a0c20 - "xwayland: clear pixmaps after creation in rootless
                   mode"
Closes: xorg/xserver#933
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>