drm/amd/display: Simplify brightness initialization

[Why]
Remove the brightness cache in DC. It uses a single value to represent
the brightness for both SDR and HDR mode. This leads to flash in HDR
on/off. It also unconditionally programs brightness as in HDR mode. This
may introduce garbage on SDR mode in miniLED panel.

[How]
Simplify the initialization flow by removing the DC cache and taking
what panel has as default. Expand the mechanism for PWM to DPCD Aux to
restore cached brightness value generally.

Cc: stable@vger.kernel.org # 6.1+
Reviewed-by: Krunoslav Kovac <krunoslav.kovac@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Camille Cho <camille.cho@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>