drm: Refresh the explanation of debug categories

That comment wasn't super-readable, so I tried to improve it:

- Put the comment before the values it's documenting
- Add a mention to PRIME
- Reword things a bit to be a lighter read
- Add a note about the option to set the debug value at run-time

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>