gpu: nvgpu: fix patch buf count update for vidmem

gr_gk20a_ctx_patch_write_begin() updates the patch buffer data_count
when the associated graphics context memory buffer has been
CPU-mapped; it was doing so by looking for a non-null cpu_va.

However, if the graphics context has been allocated from vidmem,
cpu_va is always 0, so we can't tell if nvgpu_mem_begin() was called
for the context buffer or not.

Instead:
- add a cpu_accessible flag to the nvgpu_mem struct and set
  it in nvgpu_mem_begin()

- return the value of that flag in nvgpu_mem_cpu_accessible()

- gr_gk20a_ctx_patch_write_begin() now calls this new function
  instead of checking cpu_va.

Bug 2012077
JIRA ESRM-74

Change-Id: I8401699f30b4ae7154111721c25c7ec3ff95d329
Signed-off-by: Peter Daifuku <pdaifuku@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1587293
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/include/nvgpu/nvgpu_mem.h

@@ -122,6 +122,7 @@ struct nvgpu_mem {
 	size_t					 aligned_size;
 	u64					 gpu_va;
 	bool					 skip_wmb;
+	bool					 cpu_accessible;
 
 	/*
 	 * Set when a nvgpu_mem struct is not a "real" nvgpu_mem struct. Instead
@@ -210,6 +211,15 @@ static inline bool nvgpu_mem_is_valid(struct nvgpu_mem *mem)
 
 }
 
+/*
+ * Returns true if the passed nvgpu_mem can be accessed by the CPU by virtue
+ * of having called nvgpu_mem_begin successfully.
+ */
+static inline bool nvgpu_mem_cpu_accessible(struct nvgpu_mem *mem)
+{
+	return mem->cpu_accessible;
+}
+
 /*
  * Create a nvgpu_sgt of the default implementation
  */