drm/i915: Fix pread/pwrite to work with new locking rules.

We are removing obj->mm.lock, and need to take the reservation lock
before we can pin pages. Move the pinning pages into the helper, and
merge gtt pwrite/pread preparation and cleanup paths.

The fence lock is also removed; it will conflict with fence annotations,
because of memory allocations done when pagefaulting inside copy_*_user.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
[danvet: Pick the older version to avoid the conflicts]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210128162612.927917-31-maarten.lankhorst@linux.intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20210323155059.628690-31-maarten.lankhorst@linux.intel.com