drm/i915/dp: Fix readback for target_rr in Adaptive Sync SDP

drm/i915/dp: Fix readback for target_rr in Adaptive Sync SDP

Correct the bit-shift logic to properly readback the 10 bit target_rr from
DB3 and DB4.

v2: Align the style with readback for vtotal. (Ville)

Fixes: 12ea89291603 ("drm/i915/dp: Add Read/Write support for Adaptive Sync SDP")
Cc: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260511123218.1589830-2-ankit.k.nautiyal@intel.com
(cherry picked from commit f7abc4af2b19240a145a221461dfe756cc01d74a)
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>