drm/i915: simplify check for I915G/I945G in bit 6 swizzling detection

Commit c9c4b6f6

 ("drm/i915: fix swizzle detection for gen3") added a
complicated check for I915G/I945G. Pineview and other gen3 devices match
IS_MOBILE() anyway. Simplify.

Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481627459-488-1-git-send-email-jani.nikula@intel.com