drm/i915/psr: Set frames before SU entry for psr2

Set frames before SU entry value for max resync frame count of
dpcd register 2009, bit field 0:3.

v2 :
 - add macro  EDP_PSR2_FRAME_BEFORE_SU (Rodrigo)
 - remove EDP_FRAMES_BEFORE_SU_ENTRY (Rodrigo)
 - add check ==1 for dpcd_read call (ville)

v3 : (Rodrigo)
 - move macro EDP_PSR2_FRAME_BEFORE_SU after EDP_PSR2_FRAME_BEFORE_SU
 - replace with &=

v4 :
 - change the macro to shift value (jani)
 - updated register names

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
CC: Puthikorn Voravootivat <puthik@chromium.org>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Vathsala Nagaraju <vathsala.nagaraju@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1506419953-32605-2-git-send-email-vathsala.nagaraju@intel.com