drm_hwcomposer: Drop obsolete drm_gralloc include path

drm_hwcomposer: Drop obsolete drm_gralloc include path

In commit d12274d, "drm_hwcomposer: Rework platformdrmgeneric.cpp
to use libdrm's gralloc handle", the use of drm_gralloc was removed.

Cc: John Stultz <john.stultz@linaro.org>
Cc: Rob Herring <rob.herring@linaro.org>
Cc: Sean Paul <seanpaul@google.com>
Cc: Robert Foss <robert.foss@collabora.com>
Cc: Stefan Schake <stschake@gmail.com>
Acked-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Alistair Strachan <astrachan@google.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>