drm/vmwgfx: Fix incorrect enum usage

SVGA_REG_ENABLE is a register name, and SVGA_REG_ENABLE_(ENABLE|
DISABLE|HIDE) are its valid values. We were incorrectly setting
the register value to itself. This happened to work because
the SVGA_REG_ENABLE is happens to to be the same value as
SVGA_REG_ENABLE_ENABLE, but is still semantically incorrect.
Lets use the correct enum when setting SVGA_REG_ENABLE.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210505035740.286923-2-zackr@vmware.com