[Intel-gfx] [RFC] Remove cause of 'primary plane already enabled' kernel warning

[joao.santos at intel.com]

I keep seeing a warning coming from intel_enable_primary_plane() in the kernel
log. It seems someone overzelous set the primary_plane flag before calling the 
function and that causes the assert to trigger. It only seems to be polluting
log output.

I managed to boot and run a gl benchmark without getting any warnings with
this patch. Pls comment and let me know if there is more to this than 
I could see.

Thanks,
Joao.