drm/i915/ddi: Move platform checks within mtl_ddi_enable/disable_d2d_link()

The prefix of the mtl_ddi_enable_d2d() / mtl_ddi_disable_d2d_link()
names show already what are the relevant platforms, so the corresponding
platform check is a detail that can be hidden in the functions, do so.

While at it rename mtl_ddi_disable_d2d_link() to mtl_ddi_disable_d2d()
for symmetry with mtl_ddi_enable_d2d().

v2: s/mtl_ddi_disable_d2d_link/mtl_ddi_disable_d2d (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250214142001.552916-9-imre.deak@intel.com