drm/i915/psr: Add helpers for block count number handling

Add helpers to make it more clear how PSR2_CTL[Block Count Number]
is configured.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230329150703.432072-5-jouni.hogander@intel.com