[Mesa-dev] Enable HW GenerateMipmap for radeon v2

[Will Dyson]

Wire up _mesa_meta_GenerateMipmap() for radeon. This greatly improves the
playability of some games, such as FlightGear and the recently GPL'd game
Lugaru.

FlightGear still generates many fallbacks due to Alpha-only textures, but
only at startup time.

V2:
Address comments from Maciej Cencora <m.cencora at gmail.com>.
Fixed meta.h inclusion.
Added comment where MaxLevel test is changed. Reword other comments.
Provide an upper bound check for the image level.
Don't gratuitiously move code around.