drm/i915/psr: set CHICKEN_TRANS for psr2

As per bpsec, CHICKEN_TRANS_EDP bit 12 ,15 must be programmed in
psr2 enable sequence.
bit 12 : Program Transcoder EDP VSC DIP header with a valid setting for
        PSR2 and Set CHICKEN_TRANS_EDP(0x420cc) bit 12 for programmable
        header packet.
bit 15 : Set CHICKEN_TRANS_EDP(0x420cc) bit 15 if Y coordinate is supported

v2: (Rodrigo)
- move CHICKEN_TRANS_EDP bit set logic right after setup_vsc

v3:(Rodrigo)
- initialize chicken_trans to CHICKEN_TRANS_BIT12 instead of 0

v4:(chris wilson)
- use BIT(12), remove CHICKEN_TRANS_BIT12
- remove unnecessary comments
- update commit message

v5:
- rename bit 12 PSR2_VSC_ENABLE_PROG_HEADER
- rename bit 15 PSR2_ADD_VERTICAL_LINE_COUNT

v6:(Rodrigo)
- remove TRANS_EDP=3, use cpu_transcoder

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/1484247691-20930-1-git-send-email-vathsala.nagaraju@intel.com