drm/amdgpu/display/dm: add missing parameter documentation

Added a new parameter and forgot to update the documentation.

Fixes: b6f91fc1 ("drm/amdgpu/display: buffer INTERRUPT_LOW_IRQ_CONTEXT interrupt work")
Reviewed-by: Xiaogang Chen <xiaogang.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Xiaogang Chen <xiaogang.chen@amd.com>