drm/i915/psr: report live PSR2 State

Reports  live state of PSR2 form PSR2_STATUS register.
bit field 31:28 gives the live state of PSR2.
It can be used to check if system is in deep sleep,
selective update or selective update standby.
During video play back, we can use this to check
if system is entering SU mode or not.
when system is in idle state, DEEP_SLEEP(8) must be entered.
When video playback is happening, system must be in
SLEEP(3 / selective update) or SU_STANDBY( 6 / selective update standby)

v2: (Rodrigo)
- Remove EDP_PSR2_STATUS_TG_ON=a ,instead use ARRAY_SIZE

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Vathsala Nagaraju <vathsala.nagaraju@intel.com>
Signed-off-by: Patil Deepti <deepti.patil@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1483720352-24761-1-git-send-email-vathsala.nagaraju@intel.com