drm/dsi: rename MIPI_DCS_SET_PARTIAL_AREA to MIPI_DCS_SET_PARTIAL_ROWS

The DCS command has been named SET_PARTIAL_ROWS in the DCS spec since
v1.02, for more than a decade. Rename the enumeration to match the spec.

v2: add comment about the rename (David Lechner)

Cc: David Lechner <david@lechnology.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191028150047.22048-4-jani.nikula@intel.com