gpu: nvgpu: Remove user_mapped from mapped_buf

Remove the always true field 'user_mapped' from the mapped_buf
struct. All mapped_bufs are mapped by a user request since they
always originate from a dma_buf (for Linux, that is). As such
there is a fair amount of logic that could be deleted.

Linux specific: the own_mem_ref field was also be deleted. The
logic of only storing a dma_buf ref when the buffer is mapped
for the first time by a user is easy: when the mapped buffer is
found in the map cache release the outstanding dma_buf ref taken
earlier on in the map path. If the map cache does not have the
buffer simply let the higher level map code keep the dma_buf ref.

The dma_buf ref is released when the nvgpu_vm_unmap_system()
call-back is called by the unmap path.

JIRA NVGPU-30
JIRA NVGPU-71

Change-Id: I229d136713812a7332bdadd5ebacd85d983bbbf0
Signed-off-by: Alex Waterman <alexw@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1583983
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/include/nvgpu/linux/vm.h

@@ -72,7 +72,6 @@ u64 nvgpu_vm_map_linux(struct vm_gk20a *vm,
 			*/
 		       s16 incompr_kind,
 
-		       bool user_mapped,
 		       int rw_flag,
 		       u64 buffer_offset,
 		       u64 mapping_size,