drm/i915/pmu: Fix build error with GCOV and AutoFDO enabled

i915_pmu.c may fail to build with GCOV and AutoFDO enabled.

../drivers/gpu/drm/i915/i915_pmu.c:116:3: error: call to '__compiletime_assert_487' declared with 'error' attribute: BUILD_BUG_ON failed: bit > BITS_PER_TYPE(typeof_member(struct i915_pmu, enable)) - 1
116 | BUILD_BUG_ON(bit >
| ^

Here is a way to reproduce the issue:
$ git checkout v6.15
$ mkdir build
$ ./scripts/kconfig/merge_config.sh -O build -n -m <(cat <<EOF
CONFIG_DRM=y
CONFIG_PCI=y
CONFIG_DRM_I915=y

CONFIG_PERF_EVENTS=y

CONFIG_DEBUG_FS=y
CONFIG_GCOV_KERNEL=y
CONFIG_GCOV_PROFILE_ALL=y

CONFIG_AUTOFDO_CLANG=y
EOF
)
$ PATH=${PATH}:${HOME}/llvm-20.1.5-x86_64/bin make LLVM=1 O=build \
olddefconfig
$ PATH=${PATH}:${HOME}/llvm-20.1.5-x86_64/bin make LLVM=1 O=build \
CLANG_AUTOFDO_PROFILE=...PATH_TO_SOME_AFDO_PROFILE... \
drivers/gpu/drm/i915/i915_pmu.o

Although not super sure what happened, by reviewing the code, it should
depend on `__builtin_constant_p(bit)` directly instead of assuming
`__builtin_constant_p(config)` makes `bit` a builtin constant.

Also fix a nit, to reuse the `bit` local variable.

Fixes: a644fde77ff7 ("drm/i915/pmu: Change bitmask of enabled events to u32")
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20250612083023.562585-1-tzungbi@kernel.org