drm/i915/bxt: DSI encoder support in CRTC modeset

SKL and BXT qualifies the HAS_DDI() check, and hence haswell
modeset functions are re-used for modeset sequence. But DDI
interface doesn't include support for DSI.
This patch adds:
1. cases for DSI encoder, in those modeset functions and allows
   a CRTC modeset
2. Adds call to pre_pll enabled from CRTC modeset function. Nothing
   needs to be done as such in CRTC for DSI encoder, as PLL, clock
   and and transcoder programming will be taken care in encoder's
   pre_enable and pre_pll_enable function.

v2: Fixed Jani's review comments. Added INVALID_PORT for non DDI
    encoder like DSI for platforms having HAS_DDI as true.

v3: Rebased on latest drm-nightly branch. Added a WARN_ON for invalid
    encoder.

v4: WARN_ON for invalid encoder is refactored as per Jani's suggestion.
    Fixed the sequence for pre_pll_enable.

v5: Protected DDI code paths in case of DSI encoder calls.

Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>