isl: render target cube maps should be handled as 2D images, not cubes

isl: render target cube maps should be handled as 2D images, not cubes

This fixes layered rendering Vulkan CTS tests with cube (arrays). We
also do this in the GL driver, see this code from gen8_depth_state.c
for example:

case GL_TEXTURE_CUBE_MAP_ARRAY:
case GL_TEXTURE_CUBE_MAP:
   /* The PRM claims that we should use BRW_SURFACE_CUBE for this
    * situation, but experiments show that gl_Layer doesn't work when we do
    * this.  So we use BRW_SURFACE_2D, since for rendering purposes this is
    * equivalent.
    */
   surftype = BRW_SURFACE_2D;
   depth *= 6;
   break;

So I guess we simply forgot to port this workaround to Vulkan.

v2: tweak the conditions so the special case is cube texture sampling
    rather than anything else (Jason)

Fixes:
dEQP-VK.geometry.layered.cube*

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>