libc2dcolorconvert: fix arithmetic on a pointer to void

Summary:
Use char* instead of void* for this
pointer arithmetic. Fixes the following:

hardware/qcom/media/libc2dcolorconvert/C2DColorConverter.cpp:321:70: error: arithmetic on a pointer to void
        srcSurfaceDef->phys0  = getMappedGPUAddr(fd, data, mSrcSize) + ((uint8_t *)data - (uint8_t *)base);
                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
hardware/qcom/media/libc2dcolorconvert/C2DColorConverter.cpp:333:70: error: arithmetic on a pointer to void
        dstSurfaceDef->phys0  = getMappedGPUAddr(fd, data, mDstSize) + ((uint8_t *)data - (uint8_t *)base);

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D134