drm/radeon: 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.

This change also allows to support prime-based mmap via DRM's helper
drm_gem_prime_mmap().

Permission checks are implemented by drm_gem_mmap(), with an additional
check for radeon_ttm_tt_has_userptr() in the GEM object function. The
function radeon_verify_access() is now unused and has thus been removed.

As a side effect, radeon_ttm_vm_ops and radeon_ttm_fault() are now
implemented in amdgpu's GEM code.

v3:
	* remove unnecessary checks from mmap (Christian)
v2:
	* rename radeon_ttm_vm_ops and radeon_ttm_fault() to
	  radeon_gem_vm_ops and radeon_gem_fault() (Christian)
	* fix commit description (Alex)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210525151055.8174-4-tzimmermann@suse.de