radv/pipeline: Don't dereference NULL dynamic state pointers

radv/pipeline: Don't dereference NULL dynamic state pointers

This is a port of commit a4a59172482d50318a5ae7f99021bcf0125e0f53:

   Add guards to prevent dereferencing NULL dynamic pipeline state. Asserts
   of pCreateInfo members are moved to the earliest points at which they
   should not be NULL.

This fixes a segfault, related to pColorBlendState, seen in Talos Principle
which I've observed after startup is completed and when exiting the menus,
depending on when Vulkan rendering is selected.

v2: moved the NULL check in radv_pipeline_init_blend_state to after the
declarations.
Acked-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>