drm/msm: 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 msm functions are being removed. The file_operations
structure fops is now being created by the helper macro
DEFINE_DRM_GEM_FOPS().

v2:
	* rebase onto latest upstream
	* remove declaration of msm_gem_mmap_obj() from msm_fbdev.c

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20210706084753.8194-1-tzimmermann@suse.de


[squash in missing VM_DONTEXPAND flag]
Signed-off-by: Rob Clark <robdclark@chromium.org>