drm/amdkfd: reformat IOCTL definitions to drm-style

This patch reformats the ioctl definitions in kfd_ioctl.h to be similar to the
drm ioctls definition style.

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>