drm/i915: PLL macro cleanup and pipe assertion check

Add a macro for accessing the two pipe PLLs and add a check to make sure
we don't access a non-existent one in the enable/disable functions.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-By: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-By: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>