drm/i915: Program DSPCLK_GATE_D only once on Ironlake

With the consolidated registers, it appears that we're setting the same
bis several times. Let's just collect the bits we want to set and program
it once.

v2: More cleanup. Also program 0x42004 and 0x45000 for FBC on non
    mobile platforms (Paulo Zanoni)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Undo the functional change as discussed on irc.]
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>