drm/i915: Correct the base value while updating LP_OUTPUT_HOLD in MIPI_PORT_CTRL

LP_OUTPUT_HOLD is only in MIPI_PORT_CTRL(PORT_A) even for PORT_C in case
of dual link. In the dual link implementation, the bit is correctly set
or unset for hardcoded PORT_A, but for bit update the register base value
is read by using MIPI_PORT_CTRL(port) in a loop. The second iteration will
read base value from PORT_C and program for PORT_A. Mostly in case of dual
link all other bit values should be same, but logically we should read from
PORT_A. So hardcode to read initial value from PORT_A as well.

Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>