Hans de Goede authored Sep 17, 2016 and Adam Jackson committed Sep 28, 2016

Commit b4e46c04 ("xfree86: Hook up colormaps and RandR 1.2 gamma code")
dropped the providing of a pScrn->ChangeGamma callback from the xf86RandR12
code. Leaving pScrn->ChangeGamma NULL in most cases.

This triggers the BadImplementation error in xf86ChangeGamma() :

    if (pScrn->ChangeGamma)
        return (*pScrn->ChangeGamma) (pScrn, gamma);

    return BadImplementation;

Which causes X-apps using XF86VidModeSetGamma to crash with a
X protocol error.

This commit fixes this by re-introducing the xf86RandR12ChangeGamma
helper removed by the commit and adjusting it to work with the new
combined palette / gamma code.

Fixes: b4e46c04

 ("xfree86: Hook up colormaps and RandR 1.2 gamma code")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>