Move the specialised interactions with the physical GEM object from the
pread/pwrite ioctl handler into the phys backend.

Currently, if one is able to exhaust the entire aperture and then try to
pwrite into an object not backed by struct page, we accidentally invoked
the phys pwrite handler on a non-phys object; calamitous.

Fixes: c6790dc2 ("drm/i915: Wean off drm_pci_alloc/drm_pci_free")
Testcase: igt/gem_pwrite/exhaustion
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Cc: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20201105154934.16022-2-chris@chris-wilson.co.uk