drm/i915/dsb: DSB context creation.

This patch adds a function, which will internally get the gem buffer
for DSB engine. The GEM buffer is from global GTT, and is mapped into
CPU domain, contains the data + opcode to be feed to DSB engine.

v1: Initial version.

v2:
- removed some unwanted code. (Chris)
- Used i915_gem_object_create_internal instead of _shmem. (Chris)
- cmd_buf_tail removed and can be derived through vma object. (Chris)

v3: vma realeased if i915_gem_object_pin_map() failed. (Shashank)

v4: for simplification and based on current usage added single dsb
object in intel_crtc. (Shashank)

v5: seting NULL to cmd_buf moved outside of mutex in dsb-put(). (Shashank)

v6:
- refcount machanism added.
- Used atomic_add_return and atomic_dec_and_test instead of
atomic_inc and atomic_dec. (Jani)

Cc: Imre Deak <[email protected]>
Cc: Michel Thierry <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Cc: Shashank Sharma <[email protected]>
Signed-off-by: Animesh Manna <[email protected]>
[Jani: added #include <linux/types.h> while pushing]
Signed-off-by: Jani Nikula <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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