drm/amd/display: Add 'pstate_keepout' kdoc entry in 'optc1_program_timing'

Fixes the below with gcc W=1:
Function parameter or struct member 'pstate_keepout' not described in 'optc1_program_timing'

Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/optc/dcn10/dcn10_optc.c

@@ -148,6 +148,7 @@ void optc1_setup_vertical_interrupt2(
  * @vstartup_start: Vstartup period.
  * @vupdate_offset: Vupdate starting position.
  * @vupdate_width: Vupdate duration.
+ * @pstate_keepout: determines low power mode timing during refresh
  * @signal: DC signal types.
  * @use_vbios: to program timings from BIOS command table.
  *