gralloc: Use compile time flag for mdss.

The gralloc allocation can happen from processes other than
surfaceflinger in which case we cannot open the framebuffer
device to get the mdp version. Use the MDSS_TARGET flag to
differentiate.

Change-Id: I69bc9ff6004b382a2836274a32fbbd3b2e67dea2
Merge "gralloc: Use compile time flag for mdss." into cm-10.2