i965: Make calculate_attr_overrides return the URB read offset.

i965: Make calculate_attr_overrides return the URB read offset.

Traditionally, we've hardcoded "URB Entry Read Offset" to 1 (which
represents 2 vec4 varying slots) to skip over the 8 DWord VUE header.

In order to support ARB_fragment_layer_viewport, we'll need to read
from that header.  This patch adds the basic plumbing necessary to
calculate a value dynamically and hook it up in the SBE packets.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>