mesa/main: do not allow ARB_depth_buffer_float enums before gles3

mesa/main: do not allow ARB_depth_buffer_float enums before gles3

Floating-point depth buffers are only supported on OpenGL 3.0, OpenGL ES
3.0, or if ARB_depth_buffer_float is supported. Because we checked a
driver capability rather than using an extension-check helper, we ended
up incorrectly allowing this on OpenGL ES 1.x and 2.x.

Since this logic is repeated, let's make a helper for it.

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