drm/amdkfd: rewrite kfd_ioctl() according to drm_ioctl()

This patch changes kfd_ioctl() to be very similar to drm_ioctl().

The patch defines an array of amdkfd_ioctls, which maps IOCTL definition to the
ioctl function.

The kfd_ioctl() uses that mapping to call the appropriate ioctl function,
through a function pointer.

This patch also declares a new typedef for the ioctl function pointer.

v2: Renamed KFD_COMMAND_(START|END) to AMDKFD_...

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