meta: use _mesa_set_enable(), fix decompress_texture_image()

meta: use _mesa_set_enable(), fix decompress_texture_image()

Use _mesa_set_enable() to avoid a redudant context lookup.
Need to disable the texture target in decompress_texture_image() so the
unit isn't still enabled after glGetTexImage() returns.  Arguably, the
meta restore code should do this, but it doesn't.

Reviewed-by: Eric Anholt <eric@anholt.net>