gallium: add new pipe_screen::can_create_resource() function

gallium: add new pipe_screen::can_create_resource() function

Used to implement proxy textures.  If a gallium driver doesn't implement
this function we'll just continue to use the core Mesa fallback code.

Without this hook we really have no good way to implement OpenGL proxy
textures with gallium drivers.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>