drm/i915/ddi: add use_edp_hobl() and use_edp_low_vswing() helpers

Localize HOBL and low vswing VBT lookups to a couple of small helpers,
and get rid of a bunch of local variables.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211203131318.2885969-1-jani.nikula@intel.com