radv: Set the user SGPR MSB for Vega.

Otherwise using 32 user SGPRs would be broken.

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit d97c8925)
Conflicts resolved by Dylan

Conflicts:
	src/amd/vulkan/radv_shader.c