drm/i915: Keep overlay functions naming consistent

All other overlay functions(almost all other functions in i915)
follow intel_overlay_verb, so renaming overlay ones that do not match
that.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181108001647.11276-4-jose.souza@intel.com