drm/i915/dsi: disassociate VBT video transfer mode from register values

The VBT DSI video transfer mode field values have been defined in terms
of the VLV MIPI_VIDEO_MODE_FORMAT register. The ICL DSI code maps that
to ICL DSI_TRANS_FUNC_CONF() register. The values are the same, though
the shift is different.

Make a clean break and disassociate the values from each other. Assume
the values can be different, and translate the VBT value to VLV and ICL
register values as needed. Use the existing macros from intel_bios.h.

This will be useful in splitting the DSI register macros to files by DSI
implementation.

Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220217224023.3994777-1-jani.nikula@intel.com