drm/amd/pm: add missing prototypes to amdgpu_dpm_internal

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

drivers/gpu/drm/amd/amdgpu/../pm/amdgpu_dpm_internal.c:29:6: warning: no
previous prototype for function 'amdgpu_dpm_get_active_displays'
[-Wmissing-prototypes]
void amdgpu_dpm_get_active_displays(struct amdgpu_device *adev)
     ^
drivers/gpu/drm/amd/amdgpu/../pm/amdgpu_dpm_internal.c:29:1: note: declare
'static' if the function is not intended to be used outside of this
translation unit
void amdgpu_dpm_get_active_displays(struct amdgpu_device *adev)
^
static
drivers/gpu/drm/amd/amdgpu/../pm/amdgpu_dpm_internal.c:76:5: warning: no
previous prototype for function 'amdgpu_dpm_get_vrefresh'
[-Wmissing-prototypes]
u32 amdgpu_dpm_get_vrefresh(struct amdgpu_device *adev)
    ^
drivers/gpu/drm/amd/amdgpu/../pm/amdgpu_dpm_internal.c:76:1: note: declare
'static' if the function is not intended to be used outside of this
translation unit
u32 amdgpu_dpm_get_vrefresh(struct amdgpu_device *adev)
^
static
2 warnings generated.

Besides that, remove the duplicated prototype of the function
amdgpu_dpm_get_vblank_time in order to keep the consistency of the
headers.

Fixes: 6ddbd37f ("drm/amd/pm: optimize the amdgpu_pm_compute_clocks() implementations")
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Maíra Canal <maira.canal@usp.br>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>