drm/i915/psr: Preserve SRD_CTL bit 29 on PSR init

Bit 29 of SRD_CTL needs to have its value preserved according to the
B-Spec, so right before we write out the register we go ahead and read
the register and preserve the value of that bit before we write out
the configured register value.

v2: Spaces => tabs, minor name change, and commit message wording (Rodrigo)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jim Bride <jim.bride@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1502229094-13392-1-git-send-email-jim.bride@linux.intel.com