drm/i915: Make sure dsm_size has correct granularity

DSM granularity is 1MB so make sure we stick to that.

The address set by firmware in GEN12_DSMBASE in driver initialization
doesn't mean "anything above that and until end of lmem is part of DSM".
In fact, there may be a few KB that is not part of DSM on the end of
lmem. How large is that space is platform-dependent, but since it's
always less than the DSM granularity, it can be simplified by simply
aligning the size down.

v2: replace "1 * SZ_1M" with SZ_1M (Andrzej).
v3: reword commit message to explain why the round down is needed
    (Lucas)

Cc: Matthew Auld <matthew.auld@intel.com>
Suggested-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230202180243.23637-1-nirmoy.das@intel.com