drm/i915/icl: Read the correct Gen11 interrupt registers

Stop reading some now deprecated interrupt registers in both
debugfs and error state. Instead, read the new equivalents in the
Gen11 interrupt repartitioning scheme.

Note that the equivalent to the PM ISR & IIR cannot be read without
affecting the current state of the system, so I've opted for leaving
them out. See gen11_reset_one_iir() for more info.

v2: else if !!! (Paulo)
v3: another else if (Vinay)
v4:
  - Rebased
  - Renamed patch
  - Improved the ordering of GENs
  - Improved the printing of per-GEN info
v5: Avoid maybe-unitialized & add comment explaining the lack
    of PM ISR & IIR

Suggested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
[Paulo: fix commit message and coding style.]
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1525989595-18220-1-git-send-email-oscar.mateo@intel.com