radv: return the correct pitch for linear mipmaps on GFX10

On GFX9, the pitch of a level is always the pitch of the entire image
but not on GFX10.

This fixes graphics glithes with Halo - The Master Chief Collection.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2188


CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 824bd083)