drm: msm: dsi: Don't attempt changing voltage of switches

In some configurations the supplies are voltage switches and not LDOs,
making the set voltage call to fail. Check with the regulator framework
if the supply can change voltage before attempting.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>