drm/qxl: Remove empty qxl_gem_prime_mmap()

The function qxl_gem_prime_mmap() returns an error. The two callers
of gem_prime_mmap are drm_fbdev_fb_mmap() and drm_gem_dmabuf_mmap(),
which both already handle NULL-callbacks with an error code. So clear
gem_prime_mmap in qxl and remove qxl_gem_prime_mmap().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210624090500.8320-1-tzimmermann@suse.de