drm/gem-shmem: Set vm_ops in static initializer

Initialize default vm_ops in static initialization of the GEM SHMEM funcs,
instead of the mmap code. It's simply better style. GEM helpers will later
set a VMA's vm_ops from the default automatically.

v2:
	* also update the drivers that build upon GEM SHMEM

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220209155634.3994-2-tzimmermann@suse.de