drm/i915: s/stride/src_stride/ in the intel_remapped_plane_info struct

An upcoming patch adds a new dst_stride field to the
intel_remapped_plane_info struct, so for clarity rename the current
stride field to src_stride.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210325214808.2071517-23-imre.deak@intel.com