Documentation/gpu: Remove undocumented files from dcn-blockshubbub.h

The dchubbub.h and hubp.h do not have any meaningful documentation; for
this reason, this commit removes those files from the dcn-blocks
documentation.

Cc: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>