This fixes a printf warn from gcc:

drivers/gpu/drm/i915/intel_dsi_cmd.c: In function ‘dsi_vc_send_long’:
drivers/gpu/drm/i915/intel_dsi_cmd.c:181:2: warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 7 has type ‘size_t’ [-Wformat=]
Suggested-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

v2: Rebase due to register bit definition change.

v3: Mostly based on Ville's review comments.
 - Use size_t for length all around.
 - Reuse dsi_vc_send_short in dsi_vc_send_long.
 - Remove stale/incorrect comments.
 - Reverse special packet sent interrupt check.
 - Use DSI controller regs for reading, not adapter.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>