mesa/main: split float-texture support checking in two

mesa/main: split float-texture support checking in two

On OpenGL ES 2.0, there's separate extensions adding support for
half-float and float textures. So we need to validate the enums
separately as well.

This also prevents these enums from incorrectly being allowed on
OpenGL ES 1.x, where there's no extension that enables this in the
first place.

While we're at it, remove the pointless default-case, and the seemingly
stale fallthrough comment.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>