drm/i915: add MIPI DSI command sending routines

v2: Rebase due to register bit definition change.

v3: Mostly based on Ville's review comments.
 - Use size_t for length all around.
 - Reuse dsi_vc_send_short in dsi_vc_send_long.
 - Remove stale/incorrect comments.
 - Reverse special packet sent interrupt check.
 - Use DSI controller regs for reading, not adapter.

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