drm/i915/ddi: Simplify the port disabling via DDI_BUF_CTL

A port can be disabled only via a modeset (or during HW state
sanitization) when the port is enabled. Thus it's not required to check
the port's enabled state before disabling it. In any case if the port
happened to be disabled, the following disabling would be just a nop and
waiting for the buffer's idle state should succeed. Simplify the
disabling sequence accordingly.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250214142001.552916-7-imre.deak@intel.com