drm: annotate drm_core_check_feature() dev arg. as const

This static inline function doesn't modify any state.

Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190114085408.15933-1-emil.l.velikov@gmail.com