gpu: nvgpu: Split VIDMEM support from mm_gk20a.c

Split VIDMEM support into its own code files organized as such:

  common/mm/vidmem.c     - Base vidmem support
  common/linux/vidmem.c  - Linux specific user-space interaction
  include/nvgpu/vidmem.h - Vidmem API definitions

Also use the config to enable/disable VIDMEM support in the makefile
and remove as many CONFIG_GK20A_VIDMEM preprocessor checks as possible
from the source code.

And lastly update a while-loop that iterated over an SGT to use the
new for_each construct for iterating over SGTs.

Currently this organization is not perfectly adhered to. More patches
will fix that.

JIRA NVGPU-30
JIRA NVGPU-138

Change-Id: Ic0f4d2cf38b65849c7dc350a69b175421477069c
Signed-off-by: Alex Waterman <alexw@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1540705
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

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

@@ -25,6 +25,7 @@
 #include <nvgpu/kmem.h>
 #include <nvgpu/bug.h>
 #include <nvgpu/bus.h>
+#include <nvgpu/vidmem.h>
 
 #include "ioctl_ctrl.h"
 #include "ioctl_tsg.h"