panfrost: Add negative lod bias support

The shader-based lod bias workaround doesn't seem to need any changes for this to work.

pandecode should probably also work without change, but I haven't tested it.

Although lod_bias can be negative, min_lod and max_lod need to be positive, but I still changed them to int16_t for consistency and to prevent compilers from complaining about casting int16_t to uint16_t.