panfrost: Promote uniform registers late

panfrost: Promote uniform registers late

Rather than creating either a load or a uniform register read with a
fixed beginning offset, we always create a load and then promote to a
uniform register later. This will allow us to promote in a register
pressure aware manner.

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