drm: Add const to name field declaration in struct drm_prop_enum_list

There is no reason why the name field should not be const, but
several why it should. The struct should only be used by
drm_property_create_enum() and there the name-field from the struct
is passed to drm_property_add_enum(), which takes a const char * as
a parameter.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/5dd3b6a1e20452bd8abdcbc55d1e8d7f56262266.1496161066.git.jsarha@ti.com