drm/i915/ddi: Add a helper to enable a port

Add a helper to enable a port instead of open-coding it.

While at it rename intel_disable_ddi_buf() to intel_ddi_buf_disable()
for consistency.

v2: (Jani)
- s/intel_enable_ddi_buf/intel_ddi_buf_enable
- s/intel_disable_ddi_buf/intel_ddi_buf_disable

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250214142001.552916-11-imre.deak@intel.com