drm/i915: Update workaround documentation

There were several updates in the driver on how the workarounds are
handled since its documentation was written. Update the documentation to
reflect the current reality.

v2:
  - Remove footnote that was wrongly referenced, adding back the
    reference in the correct paragraph.
  - Remove "Display workarounds" and just mention "display IP" under
    "Other" category since all of them are peppered around the driver.

Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com> # v1
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221115192611.179981-1-lucas.demarchi@intel.com