i965: Rename __DRIScreen pointers to "dri_screen".

i965: Rename __DRIScreen pointers to "dri_screen".

I want to use "screen" as the variable name for a struct intel_screen
pointer.  This means that we can't use it for __DRIscreen pointers.

Sometimes we called it "screen", sometimes "sPriv", sometimes
"driScrnPriv", and sometimes "psp" (Pointer to Screen Private?).
The last one is particularly confusing because we use "psp" to refer to
the Gen4 PIPELINED_STATE_POINTERS packet as well.

Let's be consistent.  "dri_screen" is clear, and it's not used often
enough that I'm worried about the verbosity.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>