gallium: Redefine the max texture 2d cap from _LEVELS to _SIZE.

gallium: Redefine the max texture 2d cap from _LEVELS to _SIZE.

The _LEVELS assumes that the max is always power of two.  For V3D 4.2, we
can support up to 7680 non-power-of-two MSAA textures, which will let X11
support dual 4k displays on newer hardware.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>