drm/i915: Make intel_adjust_tile_offset() work for linear buffers

To make life less surprising we can make intel_adjust_tile_offset()
deal with linear buffers as well. Currently it doesn't seem like there's
a real need for this since only X tiling and NV12 (which would always
be tiled currently) should need it. But I've used it for some debug
hacks already so seems like a reasonable thing to have.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470821001-25272-10-git-send-email-ville.syrjala@linux.intel.com