Revert "drm: Use a flexible array member for blob property data"

Using a flexible array for the blob data was a mistake by me. It
forces all users of the blob data to cast blob->data to something
else. void* is clearly superior so let's go back to the original
scheme.

Not a clean revert as the code has moved.

This reverts commit d63f5e6b

.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180223192506.29992-1-ville.syrjala@linux.intel.com
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>