drm/i915: Add debugfs entry for DRRS

Adding a debugfs entry to determine if DRRS is supported or not

V2: [By Ram]: Following details about the active crtc will be filled
	in seq-file of the debugfs
	1. Encoder output type
	2. DRRS Support on this CRTC
	3. DRRS current state
	4. Current Vrefresh
Format is as follows:
CRTC 1:  Output: eDP, DRRS Supported: Yes (Seamless), DRRS_State: DRRS_HIGH_RR, Vrefresh: 60
CRTC 2:  Output: HDMI, DRRS Supported : No, VBT DRRS_type: Seamless
CRTC 1:  Output: eDP, DRRS Supported: Yes (Seamless), DRRS_State: DRRS_LOW_RR, Vrefresh: 40
CRTC 2:  Output: HDMI, DRRS Supported : No, VBT DRRS_type: Seamless

V3: [By Ram]: Readability is improved.
	Another error case is covered [Daniel]

V4: [By Ram]: Current status of the Idleness DRRS along with
	the Front buffer bits are added to the debugfs. [Rodrigo]

V5: [By Ram]: Rephrased to make it easy to understand.
	And format is modified. [Rodrigo]

V6: [By Ram]: Modeset mutex are acquired for each crtc along with
	renaming the Idleness detection states  [Daniel]

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
[danvet: dump full busy_frontbuffer_bits and remove the dubios
computed logical state of DRRS - debugfs is about what is fact,
developers should reach their own conclusion when debugging issues.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>