drm/i915: make new intel_tc.c use uncore accessors

Let's make the just created intel_tc.c already follow the trend of using
i915 instead of dev_priv and calling the intel_uncore_*() functions.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708172815.6814-2-lucas.demarchi@intel.com