drm/i915/psr: Silence period and lfps half cycle

Add get function for silence period and lfps half cycle. Values are taken
from the tables in bspec.

Bspec: 71632

v3:
  - use PORT_ALPM_CTL_SILENCE_PERIOD_MASK instead of value 255
  - use PORT_ALPM_LFPS_CTL_LAST_LFPS_HALF_CYCLE_DURATION_MASK instead of
    value 31
v2:
  - fix some checks
  - add some more comments

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240328141928.1311284-4-jouni.hogander@intel.com