drm/i915: Remove duplicated dpcd write on hsw_psr_enable_sink.

Commit (89251b17) intended to remove this line and let only one
DP_PSR_EN_CFG set, but it was wrong and this call is now duplicated
at the code.

Also "& ~DP_PSR_MAIN_LINK_ACTIVE" doesn't do anything at all. It
was like that since I introduced this call but probably the idea
was to be informative and make clear statement that we were not using
the link standby. So it is better to remove this one here and let
the code a bit cleaner.

v2: Improve commit message as requested by Paulo.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>