mesa/main: clean up OES_texture_float_linear check

mesa/main: clean up OES_texture_float_linear check

Using the _mesa_has_FOO_bar helpers is generally more safe and should
generally be prefered over checking driver-caps like this code did,
because the _mesa_has_FOO_bar helpers also verify the API type and
version.

This shouldn't have any practical effect here, as this function only
gets called for OpenGL ES 3.x right now. But if this was to change in
the future, this makes the function behave a lot more predictable.

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