drm/i915: Disable LTTPR support when the DPCD rev < 1.4

By the specification the 0xF0000-0xF02FF range is only valid when the
DPCD revision is 1.4 or higher. Disable LTTPR support if this isn't so.

Trying to detect LTTPRs returned corrupted values for the above DPCD
range at least on a Skylake host with an LG 43UD79-B monitor with a DPCD
revision 1.2 connected.

v2: Add the actual version check.
v3: Fix s/DRPX/DPRX/ typo.

Fixes: 7b2a4ab8 ("drm/i915: Switch to LTTPR transparent mode link training")
Cc: <stable@vger.kernel.org> # v5.11
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210317190149.4032966-1-imre.deak@intel.com