drm/i915/dsi: abstract dsi bpp derivation from pixel format

Nuke three copies of the same switch case.

Hopefully we can switch to a drm generic function later on, but that
will require us to swich to enum mipi_dsi_pixel_format first.

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