drm: add mmap() to drm_gem_object_funcs

drm_gem_object_funcs->vm_ops alone can't handle everything which needs
to be done for mmap(), tweaking vm_flags for example.  So add a new
mmap() callback to drm_gem_object_funcs where this code can go to.

Note that the vm_ops field is not used in case the mmap callback is
present, it is expected that the callback sets vma->vm_ops instead.

Also setting vm_flags and vm_page_prot is the job of the new callback.
so drivers have more control over these flags.

drm_gem_mmap_obj() will use the new callback for object specific mmap
setup.  With this in place the need for driver-speific fops->mmap
callbacks goes away, drm_gem_mmap can be hooked instead.

drm_gem_prime_mmap() will use the new callback too to just mmap gem
objects directly instead of jumping though loops to make
drm_gem_object_lookup() and fops->mmap work.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20191016115203.20095-2-kraxel@redhat.com