gl-renderer: Assume num images the same as the no of planes

Potential failures when creating the EGL image could cause an incorrect
number of num images (num_planes > num_images). With this change
egl_image_unref() requires an additional check to avoid any potential NULL
derefs when cleaning up. We do it straight in egl_image_unref() instead
of adding guards all over the necessary parts.

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