drm/msm: Fix possible overflow issue in submit_cmd

When verifying that the submit_cmd offset and size do not exceed the
bounds of the GEM object make sure to cast the math operation
into a suitably large buffer to account for overflow.

Change-Id: Ic0dedbad97513ee538d539e771038b3cf0405e91
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>
Signed-off-by: Venkateswara Rao Tadikonda <vtadik@codeaurora.org>