drm/amdgpu: change function declarations and add missing header dependencies

We get a few warnings when building kernel with W=1:
drivers/gpu/drm/amd/amdgpu/atombios_crtc.c:38:6: warning: no previous prototype for 'amdgpu_atombios_crtc_overscan_setup' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/dce_v8_0.c:661:6: warning: no previous prototype for 'dce_v8_0_disable_dce' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c:40:5: warning: no previous prototype for 'amdgpu_gfx_scratch_get' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c:62:6: warning: no previous prototype for 'amdgpu_gfx_scratch_free' [-Wmissing-prototypes]
....

In fact, these functions are declared in
drivers/gpu/drm/amd/amdgpu/atombios_crtc.h
drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.h
drivers/gpu/drm/amd/amdgpu/dce_v8_0.h
drivers/gpu/drm/amd/amdgpu/dce_v10_0.h
drivers/gpu/drm/amd/amdgpu/dce_v11_0.h
drivers/gpu/drm/amd/powerplay/inc/pp_acpi.h.
So this patch adds missing header dependencies.

By the way, this patch changes declaration of amdgpu_gfx_parse_disable_cu()
to subject to its implement, and clean three function declarations
in pp_acpi.h up.

Acked-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>