drm/i915: Unalias obj->phys_handle and obj->userptr

We use obj->phys_handle to choose the pread/pwrite path, but as
obj->phys_handle is a union with obj->userptr, we then mistakenly use
the phys_handle path for userptr objects within pread/pwrite.

Testcase: igt/gem_userptr_blits/forbidden-operations
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97519


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161003124516.12388-2-chris@chris-wilson.co.uk


(cherry picked from commit 5f12b80a)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>