drm/i915: don't read LVDS regs at compute_config time

We may reach this point while the machine is still runtime suspended,
so we'll hit a WARN. The other encoders also don't touch registers at
this point, so instead of waking the machine up, write some code to
keep the register always at the same state, including after we runtime
suspend/resume.

Testcase: igt/pm_rpm
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80463


Signed-off-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>