gpu: nvgpu: Use a callback to free struct gk20a

struct gk20a is now part of nvgpu_os_linux in Linux builds. gk20a.c
still frees struct gk20a by kfree(struct gk20a *), which is wrong.
Create a new function pointer in struct gk20a for freeing the
structure and call kfree(struct nvgpu_os_linux *) instead.

JIRA NVGPU-259

Change-Id: I412ee993002cb2a42f0db015fc676de43418ec2f
Signed-off-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1591012
GVS: Gerrit_Virtual_Submit
Reviewed-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/common/linux/pci.c

@@ -37,6 +37,7 @@
 #endif
 
 #include "os_linux.h"
+#include "driver_common.h"
 
 #define PCI_INTERFACE_NAME "card-%s%%s"
 
@@ -457,6 +458,7 @@ static int nvgpu_pci_probe(struct pci_dev *pdev,
 	}
 
 	g = &l->g;
+	nvgpu_init_gk20a(g);
 
 	nvgpu_kmem_init(g);