radv: Only use the gfx mipmap level offset/pitch for linear textures.

The tiled-case is non-sensical for non-base mips, but Vulkan requires
that this function handles it but at the same time does not require
returning anything useful. So we can basically return anything.

Correct tiled pitch and offset are still required for our own WSI and
in the future getting the layouts of images with DRM format modifiers.
Both don't have to deal with images with more than 1 level though.

Fixes: 824bd083 "radv: return the correct pitch for linear mipmaps on GFX10"
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2301
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2304


Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 17741a0a)