drm/amdkfd: Use order_base_2 to get log2 of buffes sizes

Replace (ffs(size) - 1) with order_base_2(size) as a more straight
forward way to get log2 of buffer sizes.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>