meta: Always bind the texture

meta: Always bind the texture

We may have been called from glGenerateTextureMipmap with CurrentUnit
still set to 0, so we don't know when we can skip binding the texture.
Assume that _mesa_BindTexture will be fast if we're rebinding the same
texture.

v2: Remove currentTexUnitSave because it is now unused.  Suggested by
both Neil and Anuj.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91847
Cc: "11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Neil Roberts <neil@linux.intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>