drm/i915/gem: Delete unused code

Unused as of commit 9e0f9464 ("drm/i915/gem: Async GPU relocations
only"), but left behind.

>> drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:933:21: error: unused function 'unmask_page' [-Werror,-Wunused-function]
static inline void *unmask_page(unsigned long p)
^
>> drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:938:28: error: unused function 'unmask_flags' [-Werror,-Wunused-function]
static inline unsigned int unmask_flags(unsigned long p)
^
>> drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:945:33: error: unused function 'cache_to_ggtt' [-Werror,-Wunused-function]
static inline struct i915_ggtt *cache_to_ggtt(struct reloc_cache *cache)

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605200357.13069-1-chris@chris-wilson.co.uk