drm/i915/gen9: minimum scanlines for Y tile is not always 4

During watermarks calculations, this value is used in 3 different
places. Only one of them was not using a hardcoded 4. Move the code up
so everybody can benefit from the actual value.

This should only help on situations with Y tiling + 90/270 rotation +
1 or 2 bpp or NV12.

Cc: stable@vger.kernel.org
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-6-git-send-email-paulo.r.zanoni@intel.com


(cherry picked from commit 1186fa85)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>