drm/i915: Fix page table entries for Bay Trail.

On Bay Trail, bit 1 means "writeable by the GPU."  Failing to set that
means basically anything using the GPU will cause hangs.

v2: Drop accidental inline keyword on byt_pte_encode.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Daniel Leung <daniel.leung@linux.intel.com> [v1]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>