gallium: Check pipe_screen::resource_changed before dereferencing it

It's optional, only implemented by the etnaviv driver so far.

Fixes: 501d0ede "st/mesa: call resource_changed when binding a
                     EGLImage to a texture"
Fixes: a37cf630 "gallium: add pipe_screen::resource_changed callback
                     wrappers"
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
(cherry picked from commit 34e89e4d)