drm/i915: Actually respect DSPSURF alignment restrictions

Currently intel_gen4_compute_page_offset() simply picks the closest
page boundary below the linear offset. That however may not be suitably
aligned to satisfy any hardware specific restrictions. So let's make
sure the page boundary we choose is properly aligned.

Also to play it a bit safer lets split the remaining linear offset into
x and y values instead of just x. This should make no difference for
most platforms since we convert the x and y offsets back into a linear
offset before feeding them to the hardware. HSW+ are different however
and use x and y offsets even with linear buffers, so they might have
trouble if either the x or y get too big.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>