gpu: nvgpu: move linux configs from Kconfig file

Kstable branch is getting rid of all downstream patches. This also
removes support for NVGPU build as kernel overlays will no longer be
supported.

In order to move towards a uniform out of tree build
system, nvgpu must manage the CONFIGS present in Kconfig itself and stop
relying on Kconfigs.

A new file Makefile.linux.configs is created to house these configs temporarily.
This file is included as part of the linux Makefile. Eventually the plan is to
move towards using Makefile.shared.configs.

This takes us one more step closer to having out of tree module building
for NVGPU internal builds.

With this change, kstable can still go ahead with building extmod builds for NVGPU.

This also allows downstream builds to continue as in-tree builds as long as the
overlays are set for the downstream kernels.

Bug 200617256

Change-Id: I78aae6b02521e2a07e8e74aa401ffdfaf9d8cf7c
Signed-off-by: Debarshi Dutta <ddutta@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/c/linux-nvgpu/+/2369209
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>
Reviewed-by: automaticguardword <automaticguardword@nvidia.com>
Reviewed-by: Alex Waterman <alexw@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/os/linux/module.c

@@ -1151,7 +1151,9 @@ static int gk20a_pm_unrailgate(struct device *dev)
 {
 	struct gk20a_platform *platform = dev_get_drvdata(dev);
 	int ret = 0;
+#ifdef CONFIG_DEBUG_FS
 	struct gk20a *g = get_gk20a(dev);
+#endif
 
 	/* return early if platform didn't implement unrailgate */
 	if (!platform->unrailgate)