VideoBackends:Vulkan: Raise number of command buffers

Avoid waiting for earlier submissions when we flush more often.
The vertex manager will flush more often if the game accesses the EFB
on the CPU, to give the GPU a head start.

Source/Core/VideoBackends/Vulkan/Constants.h

@@ -12,7 +12,7 @@
 namespace Vulkan
 {
 // Number of command buffers.
-constexpr size_t NUM_COMMAND_BUFFERS = 2;
+constexpr size_t NUM_COMMAND_BUFFERS = 8;
 
 // Number of frames in flight, will be used to decide how many descriptor pools are used
 constexpr size_t NUM_FRAMES_IN_FLIGHT = 2;