drm/i915/dp: Rearrange check for illegal mode and comments in mode_valid

Check for MODE_H_ILLEGAL before calculating max rates, lanes etc.
Move comments about compressed bpp U6.4 format closer to where it is used.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230427125605.487769-7-ankit.k.nautiyal@intel.com