drm/amd: Add amdgpu_hwmon_get_sensor_generic()

Many sensor function have a lot of boilerplate checks.  Move these
into a generic amdgpu_hwmon_get_sensor_generic() instead.

No intended functional changes.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>