mesa: Refactor compressed texture error checks to work with paletted textures

mesa: Refactor compressed texture error checks to work with paletted textures

This code was really broken before.  A lot of the error checks were
done much later (too late), and some of the error checks would fail.
The underlying problem is that Mesa doesn't ever keep compressed paletted
textures in their original format.  The textures are immediately
converted to some RGB or RGBA format.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=39991
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Tested-by: Jin Yang <jin.a.yang@intel.com>