drm: Add the PSR SU granularity registers offsets

Source is required to comply to sink SU granularity when
DP_PSR2_SU_GRANULARITY_REQUIRED is set in DP_PSR_CAPS,
so adding the registers offsets.

v2: Also adding DP_PSR2_SU_Y_GRANULARITY(Rodrigo)

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181204003403.23361-7-jose.souza@intel.com