drm/i915/dmabuf: Fix prime_mmap to work when using LMEM

The current implementation of i915 prime mmap only works when initializing
drm_i915_gem_object with shmem_region.
When using LMEM, drm_i915_gem_object is initialized with ttm_system_region.
In order to make prime mmap work even this case, when using LMEM
(when using ttm in i915), dma_buf_ops.mmap callback function calls
drm_gem_prime_mmap(). drm_gem_prime_mmap() of drm core calls internally
i915_gem_mmap() so that prime mmap can perform normally.
The fake offset is processed inside drm_gem_prime_mmap().

Testcase: igt/prime_mmap

Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220225131316.1433515-3-gwan-gyeong.mun@intel.com