radv/gfx9: fix block compression texture views.

This ports a fix from amdvlk, to fix the sizing for mip levels
when block compressed images are viewed using uncompressed views.

Fixes:
dEQP-VK.image.texel_view_compatible.graphic.extended*bc*

Fixes: e38685cc 'Revert "radv: disable support for VEGA for now."'
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 59515780)