egl: Fix leak of X11 pixmaps backing pbuffers in DRI3.

This is basically copied from the DRI2 destroy path.  Without this,
Raspberry Pi would quickly run out of CMA during the EGL tests in the CTS
due to all the pixmaps laying around.

Fixes: f35198ba ("egl/x11: Implement dri3 support with loader's dri3 helper")
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit b618d7ea)