pan/midgard: Dynamically allocate r26/27 for spills

pan/midgard: Dynamically allocate r26/27 for spills

This allows us to spill two 128-bit values in the same bundle, since we
have two registers we can spill with. This improves the
register allocation flexibility in programs with heavy spilling, though
unfortunately it isn't sufficient (theoretically, 3.5 128-bit values can
be spilled from 3 vector units and 2 scalar units).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>