Use the mmap callback in struct drm_gem_object_funcs to set the
VM flags. Replace a number of mmap helpers in omapdrm with their
GEM helper counterparts. Generate DRM's file-operations instance
with GEM's DEFINE_DRM_GEM_FOPS.

The omapdrm driver uses DRM's drm_gem_mmap() helper to prepare
the VMA structure. It then modifies the resulting VMA state in
its own helper omap_gem_mmap_obj(). The patch improves this by
setting up the VMA in the mmap callback in drm_gem_object_funcs,
which is called from within drm_gem_mmap().

Omapdrm's omap_gem_mmap() and omap_gem_mmap() can then be removed
from the driver. A call to drm_gem_mmap() is sufficient for the
mmap operation.

Finally, with the omap functions gone, the drivers file_ops in
omapdriver_fops can be generated with DEFINE_DRM_GEM_FOPS, which
sets DRM's default helpers.

v2:
	* detailed commit message (Javier)
	* do not set VM_PFNMAP
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Cc: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230707083422.18691-9-tzimmermann@suse.de