drm/amdgpu: 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 resolves several inconsistencies between regular mmap and
prime-based mmap. The vm_ops field in vma is now set for all mmap'ed
areas. Previously it way only set for regular mmap calls, prime-based
mmap used TTM's default vm_ops. The function amdgpu_verify_access() is
no longer being called and therefore removed by this patch.

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

v4:
	* rebased
v3:
	* rename mmap function to amdgpu_gem_object_mmap() (Christian)
	* remove unnecessary checks from mmap (Christian)
v2:
	* rename amdgpu_ttm_vm_ops and amdgpu_ttm_fault() to
	  amdgpu_gem_vm_ops and amdgpu_gem_fault() (Christian)
	* the check for kfd_bo has meanwhile been removed

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210525151055.8174-3-tzimmermann@suse.de