i965: Use drm_intel_get_aperture_sizes instead of hard-coded 2GiB
i965: Use drm_intel_get_aperture_sizes instead of hard-coded 2GiB

Systems with little physical memory installed will report less than
2GiB, and some systems may (hypothetically?) have a larger address space
for the GPU.  My IVB still reports 1534.

Signed-off-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Reviewed-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>
Cc: "10.0" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>

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