drm/i915/gvt: Fix bug in getting msg length in AUX CH registers handler

Msg length should be obtained from value written to AUX_CH_CTL register
rather than from enum type of the register.

Commit 0cad796a  ("drm/i915: Use REG_BIT() & co. for AUX CH registers")
incorrectly calculates the msg_length from reg type and yields below
warning in intel_gvt_i2c_handle_aux_ch_write():
"i915 0000:00:02.0: drm_WARN_ON(msg_length != 4)".

Fixes: 0cad796a ("drm/i915: Use REG_BIT() & co. for AUX CH registers")
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20230731112033.7275-1-yan.y.zhao@intel.comReviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/edid.c

@@ -491,7 +491,7 @@ void intel_gvt_i2c_handle_aux_ch_write(struct intel_vgpu *vgpu,
 		return;
 	}
 
-	msg_length = REG_FIELD_GET(DP_AUX_CH_CTL_MESSAGE_SIZE_MASK, reg);
+	msg_length = REG_FIELD_GET(DP_AUX_CH_CTL_MESSAGE_SIZE_MASK, value);
 
 	// check the msg in DATA register.
 	msg = vgpu_vreg(vgpu, offset + 4);