drm/i915/bios: add i915 backpointer to intel_bios_encoder_data

We'll be needing it in the future.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/026b737b122273d256f4247e9b0c84529aa391fd.1615998927.git.jani.nikula@intel.com