Add a new IGT test to validate DC3CO state

Add a subtest for DC3CO video playback case
to generate selective frame update and validate
that system stays in DC3CO state during execution.

v2: Changed PSR2 idle check to sleep check and addressed cosmetic changes.
v3: Renamed a function and restructured code.
v4: Cosmetic changes.
v5: Removed DC5 check, Platform check and a function parameter.
    Renamed a function name as per Arek and Imre's Comments.
v6: Added a new function require_dc_counter as per Arek's Comments.
    The test is now running based on time instead of the number of frames
    increased frame delay from 1 to 1.5 as per Imre's Comments.
v7: Removed psr2_active_sleep_check and corrected the switch indentation.
v8: Changed skip to assert in read_dc_counter function and changed
    IGT description

Signed-off-by: Jeevan B <jeevan.b@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Acked-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Link: https://patchwork.freedesktop.org/patch/338131/?series=66648&rev=8