mdss: dsi: Add support to control bias voltage regulator

The bias voltage regulator is controlled by wled driver
for the sharp panel on 8994. Add support to control this
regulator to fix suspend/resume issues.

Add optional "qcom,dsi-panel-bias-vreg" property to
support this on specific platforms.

Change-Id: Ifb02a0467dc190e673781490a918c12293d41d58
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>