glamor: Always destroy EGL image associated with destroyed pixmap

There were three paths that called eglDestroyImageKHR:

 * The front buffer
 * The intel driver's flip buffer
 * pixmaps under DRI3

This patch unifies the second two by having glamor_destroy_pixmap
always destroy any associaged EGL image. This allows us to stop
storing the back_pixmap pointer in glamor as that was only used to
make sure that buffer was freed at server reset time.

v2: check for valid pixmap_priv before using it in
glamor_egl_destroy_pixmap_image

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Zhigang Gong <zhigang.gong@linux.intel.com>
glamor: Remove redundant reference to screen pixmap EGL image

There's no reason to store this in the egl screen private as the
screen pixmap will always hold a reference to it anyways.

Signed-off-by: Keith Packard <keithp@keithp.com>
Revewied-by: Zhigang Gong <zhigang.gong@linux.intel.com>
glamor: Free existing EGL image when assigning new one

When reallocating the framebuffer on screen resize, the old EGL image
was getting leaked. Check for an existing EGL image and free it in
this case.

Signed-off-by: Keith Packard <keithp@keithp.com>
Revewied-by: Zhigang Gong <zhigang.gong@linux.intel.com>