drm/i915/display/psr: Handle SU Y granularity

We were only handling X and width granularity, what was causing issues
when sink had a granularity different than 4.

While at it, renaming su_x_granularity to su_w_granularity to better
match reality.

Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210616203158.118111-1-jose.souza@intel.com