drm/panfrost: Do the exception -> string translation using a table

Do the exception -> string translation using a table. This way we get
rid of those magic numbers and can easily add new fields if we need
to attach extra information to exception types.

v4:
* Don't expose exception type to userspace
* Merge the enum definition and the enum -> string table declaration
  in the same patch

v3:
* Drop the error field

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210630062751.2832545-7-boris.brezillon@collabora.com