drm/amd/pm: Fix missing prototype warning

Fix below warning
warning: no previous prototype for '__smu_get_enabled_features' [-Wmissing-prototypes]

Fixes: f141e251 ("drm/amd/pm: validate SMU feature enable message for getting feature enabled mask")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>