i965: implement indirect drawing for Gen7

i965: implement indirect drawing for Gen7

Just prior to emitting the 3DPRIMITIVE command, we load each of the
indirect registers. The values loaded are either from offsets into the
current indirect BO, or constant zero if the parameter is not used for
this draw.

Enabling use of the indirect registers is done by turning on a bit in
the first dword of the 3DPRIMITIVE command itself.

V3: - Deduplicate the common part of both indexed and nonindexed indirect
setup.
    - Just refer to the indirect bo out of the context directly.

V4: - Fix bo reference to specify the range we care about.

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