drm/fb-helper: Don't clobber the display palette when fbdev isn't bound

Perform the drm_fb_helper_is_bound() check to avoid clobbering the
display palette of some other KMS client.

While at it, fix up the locking by grabbing all modeset locks for the
duration of the fb_setcmap operation.

v2: Make a note of the locking changes in the commit message

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>