drm/i915/psr: set PSR_MASK bits for deep sleep

Program EDP_PSR_DEBUG_CTL (PSR_MASK) to enable system
to go to deep sleep while in psr2.PSR2_STATUS bit 31:28
should report value 8 , if system enters deep sleep state.

Also, EDP_FRAMES_BEFORE_SU_ENTRY is set 1 , if not set,
flickering is observed on psr2 panel.

v2: (Ilia Mirkin)
- Remove duplicate bit definition 25:27

v3: rebase

v4: rebase

v5: rebase

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: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484267484-21843-1-git-send-email-vathsala.nagaraju@intel.com