drm/i915/dsi: add drm mipi dsi host support

Add basic support for using the drm mipi dsi framework for DSI. We don't
use device tree which is pretty much required by mipi_dsi_host_register
and friends, and we don't have the kind of device model the functions
expect either. So we cheat and use it as a library to abstract what we
need: a nice, clean interface for DSI transfers. This means we will have
to be careful with what functions we call, as the driver model devices
in mipi_dsi_host and mipi_dsi_device will *not* be initialized.

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