drm/i915: Fix 852GM/GMV cdclk

It seems 852GM/GMV uses a different HPLLCC encoding than the other
85x platforms. For 852GM/GMV cdclk is always 133MHz. Try to detect that
using the PCI revision (sinc the device ID seems useless for that). I'm
not at all sure this is a good idea, but according to the specs it
should work.

v2: Rebased to the latest
v3: Rebased to the latest

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v1)
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Acked-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -6826,6 +6826,14 @@ static int i85x_get_display_clock_speed(struct drm_device *dev)
 {
 	u16 hpllcc = 0;
 
+	/*
+	 * 852GM/852GMV only supports 133 MHz and the HPLLCC
+	 * encoding is different :(
+	 * FIXME is this the right way to detect 852GM/852GMV?
+	 */
+	if (dev->pdev->revision == 0x1)
+		return 133333;
+
 	pci_bus_read_config_word(dev->pdev->bus,
 				 PCI_DEVFN(0, 3), HPLLCC, &hpllcc);