gpu: nvgpu: Move pd_cache declarations to new header

The pd_cache header declarations were originally part of the
gmmu.h header. This is not good from a unit isolation perspective
so this patch moves all the pd_cache specifics over to a new
header file: <nvgpu/pd_cache.h>.

Also a couple of static inlines that were possible when the code
was part of gmmu.h were turned into real, first class functions.
This allows the pd_cache.h header to not include the gmmu.h
header file.

Also fix an issue in the nvgpu_pd_write() function where the data
was being passed as a size_t for some reason. This has now been
changed to a u32.

JIRA NVGPU-1444

Change-Id: Iead9a0d998396d2289ffcb3b48765d770400397b
Signed-off-by: Alex Waterman <alexw@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1965271
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

userspace/units/mm/pd_cache/pd_cache.c

@@ -26,6 +26,7 @@
 
 #include <nvgpu/gk20a.h>
 #include <nvgpu/gmmu.h>
+#include <nvgpu/pd_cache.h>
 #include <nvgpu/enabled.h>
 
 #include <nvgpu/posix/kmem.h>