sna: Eliminate sna_mode_wants_tear_free()

The modparam checks performed by sna_mode_wants_tear_free() don't
generally work when the server is running as a regular user. Hence
we can't rely on them to indicate whether FBC/PSR/etc is enabled.
Also the "Panel Self-Refresh" connector property doesn't actually
exist so we can nuke that part as well. Let's just nuke the whole
thing and assume we want dirtyfb always when tearfree is not enabled.

I'll anyway want to enable FBC by default across the board soonish
so the check wouldn't really buy us much (would just exclude i830
and a few old desktop chipsets which don't have FBC hardware).

Additionally if we don't have working dirtyfb we really should
enable tearfree by default because otherwise we're going to
get horrible lag due to missing frontbuffer flushes.

Without WC mmaps we could in theory rely on the hw gtt tracking
except the kernel no longer differentiates between GTT/WC/CPU
access in its software frontbuffer tracking code so it'll just
deactivate FBC even for a GTT mmap and potentially never re-enable
it due to the missing frontbuffer flush from dirtyfb. So dirtyfb
is always needed.

v2: Rebase due to ppgtt->tear free logic

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

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