drm/fb-helper: Remove dead code in setcolreg

DRM fbdev emulation only supports pallete_color with depth == 8, and
truecolor with depth > 8. Handling depth == 16 for palettes is hence
dead code, let's remove it.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1459331485-28376-3-git-send-email-daniel.vetter@ffwll.ch