drm/i915/psr: Only handle interruptions of the transcoder in use

It was enabling and checking PSR interruptions in every transcoder
while it should keep the interruptions on the non-used transcoders
masked.

While doing this it gives us trouble on Tiger Lake if we are
reading/writing to registers of disabled transcoders since from gen12
onwards the registers are relative to the transcoder. Instead of forcing
them ON to access those registers, just avoid the accesses as they are
not needed.

v2 (Lucas):
  - Explain why we can't keep accessing all transcoders
  - Remove TODO about extending the irq handling to multiple instances:
    when/if implementing multiple instances it's pretty clear by the
    singleton psr that it needs to be extended
  - Fix intel_psr_debug_set() calling psr_irq_control() with
    psr.transcoder not set yet (from Imre). Now we only set the debug
    register right away if psr is already enabled. Otherwise we just
    record the value to be set when enabling the source.
  - Do not depend on the value of TRANSCODER_A. Just be relative to it
    (from Imre)
  - handle psr error last so we don't schedule the work before handling
    the other flags

v3:
  - Adding a warning about setting reserverd bits on EDP_PSR_IMR

Cc: Imre Deak <imre.deak@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190904213419.27547-2-jose.souza@intel.com