msm_fb: display: add mfd null pointer and mfd panel_power_on checks

These checks are necessary for the situations where display is off but
vsync ctrl enable or disable is still called.

CRs-Fixed: 462586
Change-Id: I2ee43ff323da38359f3bff28669df19b87d8343d
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>