drm/i915/display: Ensure phy is accessible on lfps configuration

Ensure phy is accessible on lfps configuration by adding
intel_cx0_phy_transaction_begin/end around it.

Fixes: 9dc61968 ("drm/i915/display: Add function to configure LFPS sending")
Suggested-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/20250722125618.1842615-4-jouni.hogander@intel.com