drm/i915/gem: Detect overflow in calculating dumb buffer size

drm/i915/gem: Detect overflow in calculating dumb buffer size

To multiply 2 u32 numbers to generate a u64 in C requires a bit of
forewarning for the compiler.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ramalingam C <ramalingam.c@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123125934.1401755-1-chris@chris-wilson.co.uk