drm: Initialize a linear gamma table by default

Code stolen from gma500.

This is just a minor bit of safety code that I spotted and figured it
might be useful if we put it into the core. This is to make the
get_gamma ioctl reflect likely reality even before the first set_gamma
ioctl call.

v2 on irc: Extend commit message per Maarten's suggestions.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459331485-28376-2-git-send-email-daniel.vetter@ffwll.ch