drm/i915/dsb: Pre allocate and late cleanup of cmd buffer

Pre-allocate command buffer in atomic_commit using intel_dsb_prepare
function which also includes pinning and map in cpu domain.

No functional change is dsb write/commit functions.

Now dsb get/put function is removed and ref-count mechanism is
not needed. Below dsb api added to do respective job mentioned
below.

intel_dsb_prepare - Allocate, pin and map the buffer.
intel_dsb_cleanup - Unpin and release the gem object.

RFC: Initial patch for design review.
v2: included _init() part in _prepare(). [Daniel, Ville]
v3: dsb_cleanup called after cleanup_planes. [Daniel]
v4: dsb structure is moved to intel_crtc_state from intel_crtc. [Maarten]
v5: dsb get/put/ref-count mechanism removed. [Maarten]
v6: Based on review feedback following changes are added,
- replaced intel_dsb structure by pointer in intel_crtc_state. [Maarten]
- passing intel_crtc_state to dsp-api to simplify the code. [Maarten]
- few dsb functions prototype modified to simplify code.
v7: added few cosmetic changes suggested by Jani and null check for
crtc_state in dsb_cleanup removed as suggested by Maarten.
v8: changed the function parameter to intel_crtc_state* of
ivb_load_lut_ext_max() from intel_crtc. [Maarten]
v9: error handling improved in _write() and prepare(). [Maarten]

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200520130737.11240-1-animesh.manna@intel.com