drivers: display: framebuffer: rework to make it self-contained

The "framebuf" driver was an incomplete driver expecting _clients_ to
implement missing functionality (i.e. init and device definition)
outside of the driver. This pattern of scattering driver code throughout
the tree is not common (if used at all). If certain drivers share
functionality, one can create a common module within the subsystem (see
e.g. ILI9XXX drivers).

The _generic_ framebuffer code was only used to implement the Intel
Multiboot framebuffer driver. This patch centralizes all the scattered
code in the subsystem and adjusts the driver name to "intel_multibootfb"
to make things clear. If there's ever another framebuffer driver that
shares code, it can be split into multiple modules.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>