ARM: dts: msm: Update msm_ext_disp string

Modify the msm_ext_disp string to match with the
sysfs path which userspace queries when the DP
cable notifcation is send to userspace.

Change-Id: Ic72e6d2e01bf5ec1b5fdaa2aaee159e63eb6669f
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>