venus: split up the generated headers and the driver source files
As brought up in !5800 (comment 561976) by @kusma and !5800 (comment 850482) by @gerddie, there are desires to split up the generated headers and the driver source files. This MR does exactly that.
Except for the first two commits, the rest of the commits moves the code around and fixes up includes. The first commit uses https://gitlab.freedesktop.org/olv/venus-protocol/-/commit/2564dba1f170e63b8561ce0ab2ea16fb627e92bc to re-generate the headers.
I tested this against virglrenderer with old headers, to make sure the serialization is indeed unchanged.