drm/i915/glk: Split GLK DSI device ready functionality

This patch divides glk_dsi_device_ready() function into
two part. First part will program LP wake and MIPI DSI mode
to MIPI_CTRL reg using newly defined function glk_dsi_enable_io().
glk_dsi_enable_io() will be called from intel_dsi_pre_enable.
Second part will do remaining device ready activities using
the existing function glk_dsi_device_ready().

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1497340095-5877-1-git-send-email-madhav.chauhan@intel.com