mesa: Replace MaxTextureLevels with MaxTextureSize.

mesa: Replace MaxTextureLevels with MaxTextureSize.

In most places (glGetInteger, max_legal_texture_dimensions), we wanted the
number of pixels, not the number of levels.  Number of levels is easily
recovered with util_next_power_of_two() and ffs().  More importantly, for
V3D we want to be able to expose a non-power-of-two maximum texture size
to cover 2x4k displays on HW that can't quite do 8192 wide.

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