drm/i915: Remove the definitions for Primary Ring Buffer

We only ever used the PRB0, neglecting the secondary ring buffers, and
now with the advent of multiple engines with separate ring buffers we
need to excise the anachronisms from our code (and be explicit about
which ring we mean where). This is doubly important in light of the
FORCEWAKE required to read ring buffer registers on SandyBridge.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>