i965/gen4: Set tile offsets to zero after depth rebase

i965/gen4: Set tile offsets to zero after depth rebase

Current logic calls intel_renderbuffer_set_draw_offset() which in
turn tries to calculate x and y offset against layer/level settings
that are against the original miptree actually having sufficient
levels/layers. This returns correctly x=0 y=0 regardless of the given
layer/level only because one calls intel_miptree_get_image_offset()
which goes and consults miptree offset table which in turn luckily
contains entries for max-mipmap levels, all initialised to zero even
in case of non-mipmapped.

This patch stops consulting the table and simply sets the draw
offsets to zero that are compatible with the single slice miptree
backing the renderbuffer.
This prepares for ISL based miptrees that calculate offsets
on-demand and do not tolerate levels beyond what the miptree has.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>