drm/vgem: Implement mmap as GEM object function

Moving the driver-specific mmap code into a GEM object function allows
for using DRM helpers for various mmap callbacks.

The respective vgem functions are being removed. The file_operations
structure vgem_driver_fops is now being created by the helper macro
DEFINE_DRM_GEM_FOPS().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210624095238.8804-1-tzimmermann@suse.de