draw_bmp: don't make strange decisions on broken iknput csp params

This checked params->color.space for being RGB. If the colorspace is
unset, this did dumb things because even if the imgfmt was a RGB one,
the colorspace was not set to RGB. This actually also happened to the
tests.

(Short-cutting RGB like this is actually wrong, since RGB could still
have strange gamma or primaries, which would warrant a full conversion.
So you'd need to check for these other parameters as well. To be fixed
later.)