drm/amd/display: Add missing prototypes to dcn201_init

Include the header with the prototype to silence the following clang
warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn201/dcn201_init.c:127:6:
warning: no previous prototype for function 'dcn201_hw_sequencer_construct'
[-Wmissing-prototypes]
    void dcn201_hw_sequencer_construct(struct dc *dc)
         ^

Signed-off-by: Maíra Canal <maira.canal@usp.br>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>