drm/i915: Remove unused intel_gmbus_set_speed() function

The intel_gmbus_set_speed() function is not used anywhere, remove it.

Note drivers/gpu/drm/gma500 has its own copy called
gma_intel_gmbus_set_speed() which is used, the intel_gmbus_set_speed()
version in the i915 code is not used at all

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211121191001.252076-1-hdegoede@redhat.com