Documentation/amdgpu: Fix duplicate declaration

Address the below kernel doc warning:

Documentation/gpu/amdgpu/display/display-manager:134:
drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:3: WARNING: Duplicate C
declaration, also defined at gpu/amdgpu/display/dcn-blocks:101.
Declaration is '.. c:struct:: mpcc_blnd_cfg'.
Documentation/gpu/amdgpu/display/display-manager:146:
drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:3: WARNING: Duplicate C
declaration, also defined at gpu/amdgpu/display/dcn-blocks:3.
Declaration is '.. c:enum:: mpcc_alpha_blend_mode'.

To address the above warnings, this commit uses the 'no-identifiers'
option in the dcn-blocks to avoid duplication with the previous use of
this function doc in the display-manager file. Finally, replaces the
deprecated ':function:' in favor of ':identifiers:'.

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>