Documentation/gpu: Adjust DCN documentation paths

When building the kernel-doc, it has the following complaints:

Documentation/gpu/amdgpu/display/dcn-blocks:23:
drivers/gpu/drm/amd/display/dc/inc/hw/hubp.h:3: WARNING: Duplicate C
declaration, also defined at gpu/amdgpu/display/dcn-blocks:3.

Declaration is '.. c:struct:: surface_flip_registers'.

Documentation/gpu/amdgpu/display/dcn-blocks:35:
drivers/gpu/drm/amd/display/dc/inc/hw/hubp.h:3: WARNING: Duplicate C
declaration, also defined at gpu/amdgpu/display/dcn-blocks:3.
Declaration is '.. c:struct:: surface_flip_registers'.

This error happened due to a copy-and-paste where the same file path was
duplicated multiple times to a different set of blocks. This commit
addresses this issue by using the correct file path.

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>