drm/i915: add port clock selection support for HSW

Multiple clocks can drive different outputs.

v2: use the port enums to access individual ports

v1 Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>