drm/i915: adjust framebuffer base address on gen4+

The tileoffset register only supports a limited offset in x/y of 4096,
so for giant screen configuration with a shared fb we wrap around.

Fix this by computing a linear offset in tiles (pages) and only use
the tileoffset register to offset within the tile.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>