drm/i915: drop redundant W=1 warnings from Makefile

Since commit a61ddb43 ("drm: enable (most) W=1 warnings by default
across the subsystem"), most of the extra warnings in the driver
Makefile are redundant. Remove them.

Note that -Wmissing-declarations and -Wmissing-prototypes are always
enabled by default in scripts/Makefile.extrawarn.

Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/490931748fa9a1dbac2bceda0c4b778240b10b58.1716471145.git.jani.nikula@intel.com