drm/fb-helper: separate the fb_setcmap helper into atomic and legacy paths

The legacy path implements setcmap in terms of crtc .gamma_set.

The atomic path implements setcmap by directly updating the crtc gamma_lut
property.

This has a couple of benefits:
- it makes the redundant fb helpers .load_lut, .gamma_set and .gamma_get
  completely obsolete. They are now unused and subject for removal.
- atomic drivers that support clut modes get fbdev support for those from
  the drm core. This includes atmel-hlcdc, but perhaps others as well?

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170713162538.22788-4-peda@axentia.se