drm/i915/psr: Panel Replay SU cap dpcd read return value

Currently return value of drm_dpcd_readb is not checked when reading sink
Panel Replay Selective Update capabilities.

Fix this and switch to drm_dpcd_read_byte.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Michał Grzelak <michal.grzelak@intel.com>
Link: https://lore.kernel.org/r/20250827060809.2461725-1-jouni.hogander@intel.com