radv: clear vertex bindings while resetting command buffer

Only vertex inputs accessed by vertex shader must have valid buffers
bound.

Signed-off-by: Józef Kucia <joseph.kucia@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Fixes: 5010436e "radv: bail out when binding the same vertex buffers"
(cherry picked from commit 24af0f13)