drm/i915/fb: add intel_framebuffer_alloc()

Add intel_framebuffer_alloc() to hide intel_bo_alloc_framebuffer(), as
that doesn't feel like the correct abstraction.

Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Maarten Lankhorst <dev@lankhorst.se>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/379c306c692c50f6af3b6f2488c213f12627954f.1756835342.git.jani.nikula@intel.com