drm/fb-helper: factor out pseudo-palette

The pseudo-palette has nothing to do with the crtc, so move it
out of the crtc loop and update the palette once, then break out
early.

Signed-off-by: Peter Rosin <peda@axenita.se>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1499164632-5582-2-git-send-email-peda@axentia.se