drm/i915/dp_mst: Simplify getting a drm_device pointer needed by to_i915()

Simplify getting a drm_device pointer when using to_i915() in
intel_dp_mst.c from the already available intel_display object, instead
of getting it from a DRM KMS object.

While at it rename dev_priv to i915, following the driver's standard
terminology.

Suggested-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250108151916.491113-5-imre.deak@intel.com