drm/i915: Fix RC6VIDS encode/decode

The RC6 VIDS has a linear ramp starting at 250mv, which means any values
below 250 are invalid. The old buggy macros tried to adjust for this to
be more flexible, but there is no need. As Dan pointed out the ENCODE
only ever has one value. The only invalid value for decode is an input
of 0 which means something is really wonky, and the cases where DECODE
are used either don't matter (debug values), or would be implicitly
correct (the check for less than 450).

This patch makes simpler, easier to read macros which are actually
correct. Maybe this patch can actually fix some bugs now.

Thanks to Dan for catching this. /me hides

Cc: stable@kernel.org
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -4272,8 +4272,8 @@
 #define   GEN6_PCODE_READ_MIN_FREQ_TABLE	0x9
 #define	  GEN6_PCODE_WRITE_RC6VIDS		0x4
 #define	  GEN6_PCODE_READ_RC6VIDS		0x5
-#define   GEN6_ENCODE_RC6_VID(mv)		(((mv) / 5) - 245) < 0 ?: 0
-#define   GEN6_DECODE_RC6_VID(vids)		(((vids) * 5) > 0 ? ((vids) * 5) + 245 : 0)
+#define   GEN6_ENCODE_RC6_VID(mv)		(((mv) - 245) / 5)
+#define   GEN6_DECODE_RC6_VID(vids)		(((vids) * 5) + 245)
 #define GEN6_PCODE_DATA				0x138128
 #define   GEN6_PCODE_FREQ_IA_RATIO_SHIFT	8