drm/i915: Specify bsd rings through exec flag

On Skylake GT3 we have 2 Video Command Streamers (VCS), which is asymmetrical.
For example, HEVC GPU commands can be only dispatched to VCS1 ring.
But userspace has no control when using VCS1 or VCS2. This patch introduces
a mechanism to avoid the default ping-pong mode and use one specific ring
through execution flag. This mechanism is usable for all the platforms
with 2 VCS rings.

The open source usage is from these two commits in vaapi/intel:
	commit 702050f04131a44ef8ac16651708ce8a8d98e4b8
	Author: Zhao, Yakui <yakui.zhao@intel.com>
	Date:   Mon Nov 17 12:44:19 2014 +0800

	    Allow the batchbuffer to be submitted with override flag

	commit a56efcdf27d11ad9b21664b4a2cda72d7f90f5a8
	Author: Zhao Yakui <yakui.zhao@intel.com>
	Date:   Mon Nov 17 12:44:22 2014 +0800

	    Add the override flag to assure that HEVC video command
		always uses BSD ring0 for SKL GT3 machine

v2: fix whitespace (Rodrigo)
v3: remove incorrect chunk that came on -collector rebase. (Rodrigo)
v4: change the comment (Zhipeng)
v5: address Daniel's comment (Zhipeng)

Signed-off-by: Zhipeng Gong <zhipeng.gong@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>