drm/i915/ddi: Fix HDMI port width programming in DDI_BUF_CTL

drm/i915/ddi: Fix HDMI port width programming in DDI_BUF_CTL

Fix the port width programming in the DDI_BUF_CTL register on MTLP+,
where this had an off-by-one error.

Cc: <stable@vger.kernel.org> # v6.5+
Fixes: b66a8abaa48a ("drm/i915/display/mtl: Fill port width in DDI_BUF_/TRANS_DDI_FUNC_/PORT_BUF_CTL for HDMI")
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250214142001.552916-3-imre.deak@intel.com