mdss: mdp: start vreg release index from PANEL_PM

Start vreg release index from PANEL_PM to avoid
release call with DSI_MAX_PM in error case. This
avoids crash in dsi_probe if panel node parsing fails.

Change-Id: I236f40851d41831fe37eea77aa83e2f4ce031a3b
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>