isl: Rework the get_intratile_offset function

isl: Rework the get_intratile_offset function

The old function tried to work in elements which isn't, strictly speaking,
a valid thing to do.  In the case of a non-power-of-two format, there is no
guarantee that the x offset into the tile is a multiple of the format
block size.  This commit refactors it to work entirely in terms of a tiling
(not a surface) and bytes/rows.

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>