drm/i915/dp: Use i915/intel connector local variables in i915_dsc_fec_support_show()

Cache the i915 specific device and connector pointers in
i915_dsc_fec_support_show().

v2:
- s/Cahce/Cache typo in commit log. (Stan)

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231006133727.1822579-5-imre.deak@intel.com