drm/i915/bios: add helper functions to check output support

These will be exposed to the rest of the driver and replace other
functions. Everything will operate on the child devices.

v2:
- Rebased, removed stray blank line
- Also abstracted intel_bios_encoder_supports_crt (Lucas)

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/2bd40ccc093796d16300742d1789d78ffac3c450.1615998927.git.jani.nikula@intel.com