drm/i915: Remove unused bits of i915_vma/active api

When reworking the code to move the eviction fence to the object,
the best code is removed code.

Remove some functions that are unused, and change the function definition
if it's only used in 1 place.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
[mlankhorst: Remove new use of i915_active_has_exclusive]
Link: https://patchwork.freedesktop.org/patch/msgid/20211216142749.1966107-2-maarten.lankhorst@linux.intel.com