meta/generate_mipmap: Work-around GLES 1.x problem with GL_DRAW_FRAMEBUFFER

meta/generate_mipmap: Work-around GLES 1.x problem with GL_DRAW_FRAMEBUFFER

GL_DRAW_FRAMEBUFFER does not exist in OpenGL ES 1.x, and since
_mesa_meta_begin hasn't been called yet, we have to work-around API
difficulties.  The whole reason that GL_DRAW_FRAMEBUFFER is used instead
of GL_FRAMEBUFFER is that the read framebuffer may be different.  This
is moot in OpenGL ES 1.x.

I have another patch series that would also fix this (by removing the
calls to _mesa_BindFramebuffer and friends), but it's not quite ready
yet... and I think it may be a bit heavy for some stable branches.
Consider this a stop-gap fix.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93215
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>