radv: fix clearing attachments in secondary command buffers

If no framebuffer is bound, get the number of samples and the
image format from the render pass.

This fixes new CTS dEQP-VK.geometry.layered.*.secondary_cmd_buffer.

Cc: 18.3 19.0 <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 5671f380)
Conflicts resolved by Dylan

Conflicts:
	src/amd/vulkan/radv_meta_clear.c