mdss: display: Correct regulator API usage

A device pointer should be passed to regulator_get() as the first
argument, and the regulator mapped to the MDSS device in the device
tree. Do this.

This also resolves the following message seen in debugfs, which
is present when regulator_get() is called with a NULL device:
<4>[ 1.145507] gdsc_mdss: Failed to create debugfs directory

Change-Id: I70a5137a31dbf82ae1e6634249f274cb16fe7a71
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>