drm/msm: Initialize mmap offset after constructing the buffer object

Only the msm driver provides its own implementation of gem_prime_mmap
from struct drm_driver. All other drivers use the drm_gem_prime_mmap()
helper.

Initialize the mmap offset when constructing the buffer object in msm
and reduce the gem_prime_mmap code to the generic helper. Prepares
msm for the removal of struct drm_driver.gem_prime_mmap.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230613150441.17720-2-tzimmermann@suse.de