drm/i915/psr: Check if source supports sink specific SU granularity

According to eDP spec, sink can required specific selective update
granularity that source must comply.
Here caching the value if required and checking if source supports
it.

v3:
- Returning the default granularity in case DPCD read fails(Dhinakaran)
- Changed DPCD error message level(Dhinakaran)

v4:
- Setting granularity to defaul when granularity read is equal to
0(Dhinakaran)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181204003403.23361-9-jose.souza@intel.com