drm/radeon: use kernel is_power_of_2 rather than local version

Follow the amdgpu change made in
76117507


and replace local radeon function with is_power_of_2().

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>