drm/i915/display: Apply workarounds during display init

Rather than applying display workarounds as part of
intel_clock_gating_init() (which in turn is confusingly called from
i915_gem_init during device probe), handle them at the point we're
actually initializing the display hardware.  This will also ensure that
these workarounds are properly applied during display initialization on
the Xe driver, which re-uses i915's display code, but does not call
i915's gem init.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230906234732.3728630-10-matthew.d.roper@intel.com