drm/i915/gvt: remove redundant -Wall

This flag is already set in the top level Makefile of the kernel.

Also, by having set CONFIG_DRM_I915_GVT, thereby appending -Wall to
ccflags, you undo all the -Wno-* cflags previously set in the Make
variable KBUILD_CFLAGS.

For example:

cc foo.c -Wall -Wno-format -Wall

resets -Wformat.

Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>