drm/tegra: gem: Use the proper size for GEM objects

If the requested buffer size wasn't a multiple of the page size, the
IOMMU code would round down the size to the next multiple of the page
size, thereby causing translation errors. To fix this we no longer pass
around the requested size but reuse the computed size of the GEM object.
This is already rounded to the next page boundary, so mapping that size
works out fine.

Signed-off-by: Thierry Reding <treding@nvidia.com>