drm/i915: Always wake the device to flush the GTT

Since we hold the device wakeref when writing through the GTT (otherwise
the writes would fail), we presumed that before the device sleeps those
writes would naturally be flushed and that we wouldn't need our mmio
read trick. However, that presumption seems false and a sleepy bxt seems
to require us to always manually flush the GTT writes prior to direct
access.

Fixes: e2a2aa36 ("drm/i915: Check we have an wake device before flushing GTT writes")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170829192546.1087-1-chris@chris-wilson.co.uk