drm/i915: Give names to the CCK_DISPLAY_CLOCK_CONTROL bits

Avoid using magic values for CCK frequency bits. Also the mask we were
using for the requested frequency was one bit too short. Fix it up.

Note: This also fixes the #define for a mask (spotted by Jesse in his
review).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Add note about mask change.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>