drm/pl111: Fix offset calculation for the primary plane.

If src_x/y were nonzero, we failed to shift them down by 16 to get the
pixel offset.  The recent CMA helper function gets it right.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: bed41005 ("drm/pl111: Initial drm/kms driver for pl111")
Reported-by: Mircea Carausu <mircea.carausu@broadcom.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170603015733.13266-1-eric@anholt.net


Reviewed-by: Sean Paul <seanpaul@chromium.org>