drm/msm/dsi-staging: add check for regulator count prior to enable

Add check to ensure regulator information has atleast one valid
regulator prior to enabling it.

Change-Id: Ib35876bd3da53faf237fde15f7fba99d3f3a213c
Signed-off-by: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>