accel/ivpu: Defer MMU root page table allocation

Defer root page table allocation and unify context init/fini functions.
Move allocation of the root page table from the file_priv_open function to
perform a lazy allocation approach during ivpu_bo_pin().

By doing so, we avoid the overhead of allocating page tables for simple
operations like GET_PARAM that do not require them.
Additionally, the MMU context descriptor table initialization has been
moved to the ivpu_mmu_context_map_page function.

This change streamlines the process and ensures that the descriptor table
is only initialized when it is actually needed.
Refactor init/fini functions to remove redundant code and make the context
management more straightforward.

Overall, these changes lead to a reduction in the time taken by the file
descriptor open operation, as the costly root page table allocation is now
avoided for operations that do not require it.

Signed-off-by: Karol Wachowski <karol.wachowski@intel.com>
Reviewed-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241017145817.121590-3-jacek.lawrynowicz@linux.intel.com