drm/i915: Nuke intel_ddi_hdmi_num_entries()

Since intel_ddi_level() now looks at the buf_trans table there's
no point in having intel_ddi_hdmi_num_entries() around. Just
roll the necessary bits of locic into
intel_ddi_hdmi_level()/intel_ddi_level().

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211001130107.1746-8-ville.syrjala@linux.intel.com