i965: Create a helper function for emitting PIPE_CONTROL writes.

i965: Create a helper function for emitting PIPE_CONTROL writes.

There are a lot of places that use PIPE_CONTROL to write a value to a
buffer (either an immediate write, TIMESTAMP, or PS_DEPTH_COUNT).
Creating a single function to do this seems convenient.

As part of this refactor, we now set the PPGTT/GTT selection bit
correctly on Gen7+.  Previously, we set bit 2 of DW2 on all platforms.
This is correct for Sandybridge, but actually part of the address on
Ivybridge and later!

Broadwell will also increase the length of these packets by 1; with the
refactoring, we should have to adjust that in substantially fewer
places, giving us confidence that we've hit them all.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>