android: simplify LOCAL_C_INCLUDES

Each of the libdrm_${hw} modules pull libdrm for linking as such:

libdrm's LOCAL_EXPORT_C_INCLUDE_DIRS are added to the includes list.
The former of which is already set to ${top} and ${top}/include/drm.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>