drm/i915: fix gpio register detection logic for BIOS without VBT

if no VBT is present, crt_ddc_bus will be left at 0, and cause us
to use that for the GPIO register offset. That's never a valid register
offset, so let the "undefined" value be 0 instead of -1.
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
[anholt: clarified the commit message a bit]

drivers/gpu/drm/i915/i915_drv.h

@@ -266,7 +266,7 @@ typedef struct drm_i915_private {
 
 	struct notifier_block lid_notifier;
 
-	int crt_ddc_bus; /* -1 = unknown, else GPIO to use for CRT DDC */
+	int crt_ddc_bus; /* 0 = unknown, else GPIO to use for CRT DDC */
 	struct drm_i915_fence_reg fence_regs[16]; /* assume 965 */
 	int fence_reg_start; /* 4 if userland hasn't ioctl'd us yet */
 	int num_fence_regs; /* 8 on pre-965, 16 otherwise */

drivers/gpu/drm/i915/intel_bios.c

@@ -283,10 +283,6 @@ parse_general_definitions(struct drm_i915_private *dev_priv,
 		GPIOF,
 	};
 
-	/* Set sensible defaults in case we can't find the general block
-	   or it is the wrong chipset */
-	dev_priv->crt_ddc_bus = -1;
-
 	general = find_section(bdb, BDB_GENERAL_DEFINITIONS);
 	if (general) {
 		u16 block_size = get_blocksize(general);

drivers/gpu/drm/i915/intel_crt.c

@@ -526,7 +526,7 @@ void intel_crt_init(struct drm_device *dev)
 	else {
 		i2c_reg = GPIOA;
 		/* Use VBT information for CRT DDC if available */
-		if (dev_priv->crt_ddc_bus != -1)
+		if (dev_priv->crt_ddc_bus != 0)
 			i2c_reg = dev_priv->crt_ddc_bus;
 	}
 	intel_output->ddc_bus = intel_i2c_create(dev, i2c_reg, "CRTDDC_A");