drm/gma500: use DRM_FB_HELPER_DEFAULT_OPS for fb_ops

This refactoring leads to real functional changes in the driver.

Now the struct psbfb_ops implements two additional members:

       .fb_setcmap     = drm_fb_helper_setcmap,
       .fb_pan_display = drm_fb_helper_pan_display,

and the struct psbfb_roll_ops implements one additional member:

       .fb_setcmap     = drm_fb_helper_setcmap,

and the struct psbfb_unaccel_ops implements two additional members:

       .fb_setcmap     = drm_fb_helper_setcmap,
       .fb_pan_display = drm_fb_helper_pan_display,

These changes are not tested.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Stefan Christ <contact@stefanchrist.eu>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1479078208-25221-19-git-send-email-contact@stefanchrist.eu