drm/i915: Match all PSR mode entry conditions before enabling it.

v2: Prefer seq_puts to seq_printf by Paulo Zanoni.
v3: small changes like avoiding calling dp_to_dig_port twice as noticed by
    Paulo Zanoni.
v4: Avoiding reading non-existent registers - noticed by Paulo
    on first psr debugfs patch.
v5: Accepting more suggestions from Paulo:
    * check sw interlace flag instead of i915_read
    * introduce PSR_S3D_ENABLED to avoid forgeting it whenever added.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Shobhit Kumar <shobhit.kumar@intel.com>
[danvet: Fix up debugfs output (spotted by Paulo) and rip out the
power well check since we really can't do that in a race-free manner,
so it's bogus.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>