drm/i915: split set pipeconf to pipeconf, pipemisc, pipe_gamma

Prep work for DSI transcoders. No functional changes.

v2: call split functions at a higher level (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/981252d5d5f82d009d73e1b2ae93d9ab7bee8de8.1458313400.git.jani.nikula@intel.com