i915g: switch rendering to mipmapped textures to (x,y) offsets

i915g: switch rendering to mipmapped textures to (x,y) offsets

Byte offsets simply don't work with tiled render targets when using
tiling bits. Luckily we can cox the hw into doing the right thing
with the DRAWING_RECT command by disabling the drawing rect offset
for the depth buffer.

Minor fixes by Jakob Bornecrantz.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Jakob Bornecrantz <wallbraker@gmail.com>
Signed-off-by: Jakob Bornecrantz <wallbraker@gmail.com>