drm/amdgpu: cleanup idh event/req for NV headers

1) drop the headers from AI in mxgpu_nv.c, should refer to mxgpu_nv.h

2) the IDH_EVENT_MAX is not used and not aligned with host side
   so drop it
3) the IDH_TEXT_MESSAG was provided in host but not defined in guest

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>