drm/amd/display: Add kernel doc to crc_rd_wrk field

[Why]
Receive warning message below:

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h:380: warning: Function
parameter or member 'crc_rd_wrk' not described in 'amdgpu_display_manager'

[How]
Add documentation for crc_rd_wrk.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>