drm/i915: Fix PSR2 selective update corruption after PSR1 setup

There is probably a issue in DMC firmwares(icl_dmc_ver1_07.bin and
kbl_dmc_ver1_04.bin at least) that causes PSR2 SU to fail after
exiting DC6 if EDP_PSR_TP1_TP3_SEL is kept in PSR_CTL, so for now
lets workaround the issue by cleaning PSR_CTL before enable PSR2.

v2:
- Updated commit description and comment to state that it may be
a DMC firmware issue (Rodrigo)
- No need to RMW, let's write 0 to PSR_CTL(Dhinakaran)

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190314230113.6571-1-jose.souza@intel.com