drm/i915/bdw: Set initial rps freq to RP1

Programming it outside of the rp0-rp1 range is considered a programming
error. Since we do not know that the previous value would actually be in
the range, program something we've read from the hardware, and therefore
know will work.

This is potentially an issue for platforms whose ranges are outside the
norms given in the programming guide (ie. early silicon)

v2: Use RP1 instead of RPn

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>