virgl: do not use SP_MAX_TEXTURE_*_LEVELS defines

virgl: do not use SP_MAX_TEXTURE_*_LEVELS defines

These macro-names are also used for softpipe, so let's avoid confusion
by avoiding them. Besides, they are just used in one place in virgl, so
let's just inline them into the place they are used instead.

While we're at it, fixup an error in the comment for the 3D version.
Mesa subtracts computes max-size by doing by 2^(n-1), which means this
should be 256 cubed, not 512 cubed. The other comments are correct.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Jakob Bornecrantz <jakob@collabora.com>