i965: get the jmp distance by instruction index

i965: get the jmp distance by instruction index

If dynamic instruction store size is enabled, while after the brw_JMPI()
and before the brw_land_fwd_jump() function, the eu instruction store
base address(p->store) may change. Thus, the safe way to reference the
jmp instruction is by index instead of by the instruction address.

v2: comments from Eric, don't change the prototype of brw_JMPI

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>