drm/i915: add basic MIPI DSI output support

This does not include any panel specific sub-encoders yet.

v2: Fix fixed mode handling (Daniel)

v3: Mostly based on Ville's review comments.
 - Fix MIPI_HS_TX_TIMEOUT.
 - DPI_ENABLE only for video mode.
 - Drop ULPS usage for now, use DEVICE_READY only.
 - Set MIPI_INIT_COUNT based on txclkesc.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>