mp_sys_display.c

The framebuffer won't be modified until graphics task gets
to another iteration of its infinite loop, so there's no reason
to still hold the drawlist during scanout.

Since this decouples drawlist release and ability to submit
another drawlist from each other, split existing st3m_gfx_pipe_full
into two functions - one that checks whether there's a free
drawlist available to take, and one that checks whether
a drawlist can be submitted for rasterization.