drm/i915: split get/set pipe timings to timings and src size

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/8d67a05eb869a7b0c4ee17c2d3b0b029de34851c.1458313400.git.jani.nikula@intel.com