drm/amd/pm: consistent approach for smartshift

create smartshift sysfs attributes from dGPU device even
on smartshift 1.0 platform to be consistent. Do not populate
the attributes on platforms that have APU only but not dGPU
or vice versa.

V2:
 avoid checking for the number of VGA/DISPLAY devices (Lijo)
 move code to read from dGPU or APU into a function and reuse (Lijo)

Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sathishkumar S <sathishkumar.sundararaju@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>