drm/i915: remove writeq ifdeffery

drm already provides fallback versions of readq and writeq.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473451373-9852-1-git-send-email-matthew.auld@intel.com