drm/i915: Convert INTEL_GEN() to DISPLAY_VER() as appropriate in intel_pm.c

Although most of the code in this file is display-related (watermarks),
there's some functions that are not (e.g., clock gating).  Thus we need
to do the conversions to DISPLAY_VER() manually here rather than using
Coccinelle.

In the near-future we'll probably want to think about moving watermark
logic out of intel_pm.c and into watermark-specific files under the
display/ directory.

v2:
 - Use new IS_DISPLAY_VER macro where appropriate.
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/20210320044245.3920043-5-matthew.d.roper@intel.com