drm/connector: Add helper to compare HDR metadata

All the drivers that support the HDR metadata property have a similar
function to compare the metadata from one connector state to the next,
and force a mode change if they differ.

All these functions run pretty much the same code, so let's turn it into
an helper that can be shared across those drivers.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>