drm/i915: abstract get config for cpu transcoder

Makes it neater to add the same for DSI transcoder. No functional
changes.

v2: rename to hsw_get_transcoder_state and add a comment about grabbing
    power reference (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/c473a73d69dcd61584419d85ff7908a8717b0594.1458313400.git.jani.nikula@intel.com