drm: Refresh the explanation of debug categories
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 <64bd3cb94f359c1a3ce68dae5e26b40578526277@intel.com>
Signed-off-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs