meta/generate_mipmap: Only modify the draw framebuffer binding in fallback_required

meta/generate_mipmap: Only modify the draw framebuffer binding in fallback_required

Previously GL_FRAMEBUFFER was used.  However, if GL_EXT_framebuffer_blit
is supported (note: it is supported by every Mesa driver), this is
*sometimes* an alias for GL_DRAW_FRAMEBUFFER (getters) and *sometimes*
an alias for *both* GL_DRAW_FRAMEBUFFER and GL_READ_FRAMEBUFFER
(setters).  As a result, the code saved one binding but modified both.
If the bindings were different, the GL_READ_FRAMEBUFFER would be
incorrect on exit.

Fixes the piglit fbo-generatemipmap-versus-READ_FRAMEBUFFER test.

Ideally this function would use DSA functions and not modify the binding
at all.  However, that would be a much more intrusive change because
_mesa_meta_bind_fbo_image would also need to be modified.
_mesa_meta_bind_fbo_image has a lot of callers.  Much of this code is
about to get a major rework due to bug #92363, so I don't think it
matters too much.  In fact, I discovered this bug while working on the
other bug.  Le bon temps!

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>