drm/i915: Check stolen memory size before calling drm_mm_init

Add check for zero usable stolen memory before calling drm_mm_init
to support configurations where stolen memory exists but is fully
reserved.

Also skip memory test in cases that usable stolen is smaller than
page size(amount mapped and used to test memory).

v2:
- skiping test if available memory is smaller than page size (Lucas)

Cc: Ville Syrjälä <[email protected]>
Cc: Daniele Ceraolo Spurio <[email protected]>
Cc: Lucas De Marchi <[email protected]>
Signed-off-by: José Roberto de Souza <[email protected]>
Signed-off-by: Steve Carbonari <[email protected]>
Reviewed-by: Lucas De Marchi <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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