drm_hwcomposer: separate C compliant header file.

Creating C compliant header file drm_hwc.h.
gralloc.tegra uses C compiler and depends only on code in drm_hwc.h.
The module doesn't depend on anything present in new hwcomposer.h.
This is required to fix the compile error.

Change-Id: Ib668334c03168aa4b72ad9e793eff323bd2e88fa
2 files changed