There are some corner cases wrt underrun when we enable
FBC with PSR2 on TGL. Recommendation from hardware is to
keep this combination disabled.

Bspec: 50422 HSD: 14010260002

v2: Added psr2 enabled check from crtc_state (Anshuman)
Added Bspec link and HSD referneces (Jose)

v3: Moved the logic to disable fbc to intel_fbc_update_state_cache
and removed the crtc->config usages, as per Ville's recommendation.

v4: Introduced a variable in fbc state_cache instead of the earlier
plane.visible WA, as suggested by Jose.

v5: Dropped an extra check for fbc in intel_fbc_enable and addressed
review comments by Jose.

v6: Move WA to end of function and added Jose's RB.
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201201190406.1752-2-uma.shankar@intel.com