drm/i915/gvt: better align the Makefile with i915 Makefile

Drop extra ccflags, drop extra intermediate variables, list object files
one per line alphabetically.

Cc: Zhi Wang <zhi.wang.linux@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/8bc0895376c077156a671e24ac6a5c75b7db4c9c.1649852517.git.jani.nikula@intel.com
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>