radv/gfx9: fix block compression texture views. (v2)

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

My original fix didn't power the clamping, but it looks like
the clamping is required to stop the sizing going too large.

Fixes:
dEQP-VK.image.texel_view_compatible.graphic.extended*bc*
Doesn't crash DOW3 anymore.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Fixes: e38685cc 'Revert "radv: disable support for VEGA for now."'
Signed-off-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit f6cc15dc)