drm/i915: add transcoder enable/disable functions

Along with assertion checks for the FDI transmitters and receivers
(including PLLs).  Modify the pipe enable function to check for FDI PLL
status as well, when driving PCH ports.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>