drm/i915: Cast remain to unsigned long in eb_relocate_vma

drm/i915: Cast remain to unsigned long in eb_relocate_vma

A recent commit in clang added -Wtautological-compare to -Wall, which is
enabled for i915 after -Wtautological-compare is disabled for the rest
of the kernel so we see the following warning on x86_64:

 ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1433:22: warning:
 result of comparison of constant 576460752303423487 with expression of
 type 'unsigned int' is always false
 [-Wtautological-constant-out-of-range-compare]
         if (unlikely(remain > N_RELOC(ULONG_MAX)))
            ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
 ../include/linux/compiler.h:78:42: note: expanded from macro 'unlikely'
 # define unlikely(x)    __builtin_expect(!!(x), 0)
                                            ^
 1 warning generated.

It is not wrong in the case where ULONG_MAX > UINT_MAX but it does not
account for the case where this file is built for 32-bit x86, where
ULONG_MAX == UINT_MAX and this check is still relevant.

Cast remain to unsigned long, which keeps the generated code the same
(verified with clang-11 on x86_64 and GCC 9.2.0 on x86 and x86_64) and
the warning is silenced so we can catch more potential issues in the
future.

Closes: https://github.com/ClangBuiltLinux/linux/issues/778
Suggested-by: Michel Dänzer <michel@daenzer.net>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200214054706.33870-1-natechancellor@gmail.com