radv: fix reported number of available VGPRs

It's a bit late to round up after an integer division.

Fixes: de889794 "radv: Implement VK_AMD_shader_info"
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Alex Smith <asmith@feralinteractive.com>
(cherry picked from commit d85fef1e)