drm/i915: rename has_edp_a() to ilk_has_edp_a()

Clarify that the name is specific to ILK+ PCH platforms.

v2: prefix the name with ilk rather than pch (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190122082307.4003-5-jani.nikula@intel.com