panfrost: Remove vertex buffer offset from its size

panfrost: Remove vertex buffer offset from its size

The offset is added to the base address, so we need to subtract it from
the size to maintain the same end address and thus prevent a buffer
overflow:

   end_address = start_address + size

   start_address' = start_address + offset
   size' = size - offset

   end_address' = start_address' + size'
                = (start_address + offset) + (size - offset)
                = (start_address + size) + (offset - offset)
                = start_address + size
                = end_address

   QED.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>