drm/amd/display: Fix vupdate_offload_work doc

drm/amd/display: Fix vupdate_offload_work doc

Fix the following warning in struct documentation:

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h:168: warning: expecting prototype for struct dm_vupdate_work. Prototype was for struct vupdate_offload_work instead

Fixes: c210b757b400 ("drm/amd/display: fix dmub access race condition")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>