compositor-drm: Don't set fb->size for non-dumb buffers

When creating a drm_fb from client (wl_buffer/dmabuf), gbm_surface, or
client buffers, set fb->size to 0. The size member is only used for dumb
buffers, where we mmap the whole buffer, and need the size recorded to
later pass to munmap.

Determining the full size of multi-planar buffers is difficult, as
auxiliary planes are not guaranteed to have a (height*stride)
allocation, e.g. if they are subsampled or if they do not contain pixel
data at all but, e.g., compression information. Single-plane tiled
buffers also often pad the buffer allocation to a multiple of tile
height, making our existing calculation incorrect.

Though it does no harm to record incorrect information, it also does
no good as we never use it; remove it in order to avoid any confusion.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>