drm/amdgpu: add missing header dependencies

We get a few warnings when building kernel with W=1:
drivers/gpu/drm/amd/amdgpu/amdgpu_pll.c:113:6: warning: no previous prototype for 'amdgpu_pll_compute' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/cz_smc.c:38:10: warning: no previous prototype for 'cz_get_argument' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/cz_smc.c:302:5: warning: no previous prototype for 'cz_smu_start' [-Wmissing-prototypes]
....

In fact, these functions are declared in
drivers/gpu/drm/amd/amdgpu/atombios_i2c.h
drivers/gpu/drm/amd/amdgpu/amdgpu_pll.h
drivers/gpu/drm/amd/amdgpu/cz_dpm.h
drivers/gpu/drm/amd/amdgpu/vi_dpm.h.
So this patch adds missing header dependencies.

Reviewed-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>