panfrost: Only allow colour blit shaders to be killed
Only a day after !11842 (merged) fixed timeouts in SuperTuxKart, my fix that initialised the blend equation caused pixel kill to be enabled more often, making this bug visible and breaking STK again.
It's possible this only affects v6, so this apitrace should be checked for whether it timeouts on v7:
Fixes timeouts in SuperTuxKart with the advanced rendering pipeline.
(I think it happens even with the GLES2 renderer, but I can't be bothered going back and fixing the commit message.)
Fixes: d0344619 ("panfrost: Set allow_forward_pixel_to_be_killed for blit")