drm/amdgpu: unwind properly in amdgpu_cs_parser_init()

The amdgpu_cs_parser_init() function doesn't clean up after itself but
instead the caller uses a free everything function amdgpu_cs_parser_fini()
on failure.  This style of error handling is often buggy.  In this
example, we call "drm_free_large(parser->chunks[i].kdata);" when it is
an unintialized pointer or when "parser->chunks" is NULL.

I fixed this bug by adding unwind code so that it frees everything that
it allocates.

I also mode some other very minor changes:
1) Renamed "r" to "ret".
2) Moved the chunk_array allocation to the start of the function.
3) Removed some initializers which are no longer needed.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reported-by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>