drm/amdkfd: Do copy_to/from_user in general kfd_ioctl()

This patch moves the copy_to_user() and copy_from_user() calls from the
different ioctl functions in amdkfd to the general kfd_ioctl() function, as
this is a common code for all ioctls.

This was done according to example taken from drm_ioctl.c

Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>