drm/i915: tweak the ordering in cpu_write_needs_clflush

For imported dma-buf objects we leave the object as cache_coherent = 0
across all platforms, which is reasonable given that have no clue what
the memory underneath is, and its not like the driver can ever manually
clflush the pages anyway (like with i915_gem_clflush_object) for such
objects. However on discrete we choose to treat cache_dirty = true as a
programmer error, leading to a warning. The simplest fix looks to be to
just change the ordering in cpu_write_needs_clflush to prevent ever
setting cache_dirty for dma-buf objects on discrete.

Fixes: d028a769 ("drm/i915/dmabuf: Fix prime_mmap to work when using LMEM")
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/5266


Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220622155919.355081-1-matthew.auld@intel.com