drm/i915: Reorganize the DSI enable/disable sequence

drm/i915: Reorganize the DSI enable/disable sequence

Basically ULPS handling during enable/disable has been moved to
pre_enable and post_disable phases. PLL and panel power disable
also has been moved to post_disable phase. The ULPS entry/exit
sequneces as suggested by HW team is as follows -

During enable time -
set DEVICE_READY --> Clear DEVICE_READY --> set DEVICE_READY

And during disable time to flush all FIFOs -
set ENTER_SLEEP --> EXIT_SLEEP --> ENTER_SLEEP

Also during disbale sequnece sub-encoder disable is moved to the end
after port is disabled.

v2: Based on comments from Ville
    - Detailed epxlaination in the commit messgae
    - Moved parameter changes out into another patch
    - Backlight enabling will be a new patch

v3: Updated as per Jani's comments
    - Removed the I915_WRITE_BITS as it is not needed
    - Moved panel_reset and send_otp_cmds hooks to dsi_pre_enable
    - Moved disable_panel_power hook to dsi_post_disable
    - Replace hardcoding with AFE_LATCHOUT

v4: Make intel_dsi_device_ready and intel_dsi_clear_device_ready static

Signed-off-by: Yogesh Mohan Marimuthu <yogesh.mohan.marimuthu@intel.com>
Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>