drm/msm/dsi: Fix bug in dsi_mgr_phy_enable

A recent commit introduces a bug in dsi_mgr_phy_enable. In the non
dual DSI mode, we reset the mdsi (master DSI) PHY. This isn't right
since master and slave DSI exist only in dual DSI mode. For the normal
mode of operation, we should simply reset the PHY of the DSI device
(i.e. msm_dsi) corresponding to the current bridge.

Usage of the wrong DSI pointer also resulted in a static checker
warning. That too is resolved with this fix.

Fixes: b62aa70a (drm/msm/dsi: Move PHY operations out of host)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>